@article{AdrianMartinezAlbertAndreetal.2017, author = {Adri{\´a}n-Mart{\´i}nez, S. and Albert, A. and Andr{\´e}, M. and Anghinolfi, M. and Anton, G. and Ardid, M. and Aubert, J.-J. and Baret, B. and Barrios-Marti, J. and Basa, S. and Bertin, V. and Biagi, S. and Bormuth, R. and Bouwhuis, M.C. and Bruijn, R. and Brunner, J. and Buto, J. and Capone, A. and Caramete, L. and Carr, J. and Chiarusi, T. and Circella, M. and Coniglione, R. and Costantini, H. and Coyle, P. and Creusot, A. and Dekeyser, I. and Deschamps, A. and De Bonis, G. and Distefano, C.}, title = {Stacked search for time shifted high energy neutrinos from gamma ray bursts with the ANTARES neutrino telescope}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, number = {1}, doi = {10.1140/epjc/s10052-016-4496-8}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181251}, pages = {10}, year = {2017}, abstract = {A search for high-energy neutrino emission correlated with gamma-ray bursts outside the electromagnetic prompt-emission time window is presented. Using a stacking approach of the time delays between reported gamma-ray burst alerts and spatially coincident muon-neutrino signatures, data from the Antares neutrino telescope recorded between 2007 and 2012 are analysed. One year of public data from the IceCube detector between 2008 and 2009 have been also investigated. The respective timing profiles are scanned for statistically significant accumulations within 40 days of the Gamma Ray Burst, as expected from Lorentz Invariance Violation effects and some astrophysical models. No significant excess over the expected accidental coincidence rate could be found in either of the two data sets. The average strength of the neutrino signal is found to be fainter than one detectable neutrino signal per hundred gamma-ray bursts in the Antares data at 90\% confidence level.}, language = {en} } @phdthesis{Kuger2017, author = {Kuger, Fabian}, title = {Signal Formation Processes in Micromegas Detectors and Quality Control for large size Detector Construction for the ATLAS New Small Wheel}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-152495}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {The Micromegas technology is one of the most successful modern gaseous detector concepts and widely utilized in nuclear and particle physics experiments. Twenty years of R \& D rendered the technology sufficiently mature to be selected as precision tracking detector for the New Small Wheel (NSW) upgrade of the ATLAS Muon spectrometer. This will be the first large scale application of Micromegas in one of the major LHC experiments. However, many of the fundamental microscopic processes in these gaseous detectors are still not fully understood and studies on several detector aspects, like the micromesh geometry, have never been addressed systematically. The studies on signal formation in Micromegas, presented in the first part of this thesis, focuses on the microscopic signal electron loss mechanisms and the amplification processes in electron gas interaction. Based on a detailed model of detector parameter dependencies, these processes are scrutinized in an iterating comparison between exper- imental results, theory prediction of the macroscopic observables and process simulation on the microscopic level. Utilizing the specialized detectors developed in the scope of this thesis as well as refined simulation algorithms, an unprecedented level of accuracy in the description of the microscopic processes is reached, deepening the understanding of the fundamental process in gaseous detectors. The second part is dedicated to the challenges arising with the large scale Micro- megas production for the ATLAS NSW. A selection of technological choices, partially influenced or determined by the herein presented studies, are discussed alongside a final report on two production related tasks addressing the detectors' core components: For the industrial production of resistive anode PCBs a detailed quality control (QC) and quality assurance (QA) scheme as well as the therefore required testing tools have been developed. In parallel the study on micromesh parameter optimization and production feasibility resulted in the selection of the proposed mesh by the NSW community and its full scale industrial manufacturing. The successful completion of both tasks were im- portant milestones towards the construction of large size Micromegas detectors clearing the path for NSW series production.}, subject = {Gasionisationsdetektor}, language = {en} } @phdthesis{Gottscholl2022, author = {Gottscholl, Andreas Paul}, title = {Optical Accessible Spin Defects in Hexagonal Boron Nitride: Identification, Control and Application of the Negatively Charged Boron Vacancy VB-}, doi = {10.25972/OPUS-27432}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-274326}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In this work, a bridge was built between the so-far separate fields of spin defects and 2D systems: for the first time, an optically addressable spin defect (VB-) in a van der Waals material (hexagonal boron nitride) was identified and exploited. The results of this thesis are divided into three topics as follows: 1.) Identification of VB-: In the scope of this chapter, the defect ,the negatively charged boron vacancy VB-, is identified and characterized. An initialization and readout of the spin state can be demonstrated optically at room temperature and its spin Hamiltonian contributions can be quantified. 2.) Coherent Control of VB-: A coherent control is required for the defect to be utilized for quantum applications, which}, subject = {Bornitrid}, language = {en} } @phdthesis{Betzold2022, author = {Betzold, Simon}, title = {Starke Licht-Materie-Wechselwirkung und Polaritonkondensation in hemisph{\"a}rischen Mikrokavit{\"a}ten mit eingebetteten organischen Halbleitern}, doi = {10.25972/OPUS-26665}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-266654}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Kavit{\"a}ts-Exziton-Polaritonen (Polaritonen) sind hybride Quasiteilchen, die sich aufgrund starker Kopplung von Halbleiter-Exzitonen mit Kavit{\"a}tsphotonen ausbilden. Diese Quasiteilchen weisen eine Reihe interessanter Eigenschaften auf, was sie einerseits f{\"u}r die Grundlagenforschung, andererseits auch f{\"u}r die Entwicklung neuartiger Bauteile sehr vielversprechend macht. Bei Erreichen einer ausreichend großen Teilchendichte geht das System in den Exziton-Polariton-Kondensationszustand {\"u}ber, was zur Emission von laserartigem Licht f{\"u}hrt. Organische Halbleiter als aktives Emittermaterial zeigen in diesem Kontext großes Potential, da deren Exzitonen neben großen Oszillatorst{\"a}rken auch hohe Bindungsenergien aufweisen. Deshalb ist es m{\"o}glich, unter Verwendung organischer Halbleiter selbst bei Umgebungsbedingungen {\"a}ußerst stabile Polaritonen zu erzeugen. Eine wichtige Voraussetzung zur Umsetzung von integrierten opto-elektronischen Bauteilen basierend auf Polaritonen ist der kontrollierte r{\"a}umliche Einschluss sowie die Realisierung von frei konfigurierbaren Potentiallandschaften. Diese Arbeit besch{\"a}ftigt sich mit der Entwicklung und der Untersuchung geeigneter Plattformen zur Erzeugung von Exziton-Polaritonen und Polaritonkondensaten in hemisph{\"a}rischen Mikrokavit{\"a}ten, in die organische Halbleiter eingebettet sind.}, subject = {Exziton-Polariton}, language = {de} } @phdthesis{Sochor2021, author = {Sochor, Benedikt}, title = {Aggregation behavior of Pluronic P123 in bulk solution and under confinement at elevated temperatures near its cloud point}, doi = {10.25972/OPUS-24607}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-246070}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {This thesis aims to investigate the form-phase diagram of aqueous solutions of the triblock copolymer Pluronic P123 focusing on its high-temperature phases. P123 is based on polyethylene as well as polypropylene oxide blocks and shows a variety of di erent temperaturedependent micelle morphologies or even lyotropic liquid crystal phases in aqueous solutions. Besides the already well-studied spherical aggregates at intermediate temperatures, the size and internal structure of both worm-like and lamellar micelles, which appear near the cloud point, is determined using light, neutron and X-ray scattering. By combining the results of time-resolved dynamic light as well as small-angle neutron and X-ray scattering experiments, the underlying structural changes and kinetics of the sphere-to-worm transition were studied supporting the random fusion process, which is proposed in literature. For temperatures near the cloud point, it was observed that aqueous P123 solutions below the critical crystallization concentration gelate after several hours, which is linked to the presence and structure of polymeric surface layers on the sample container walls as shown by neutron re ectometry measurements. Using a hierarchical model for the lamellar micelles including their periodicity as well as domain and overall size, it is possible to unify the existing results in literature and propose a direct connection between the near-surface and bulk properties of P123 solutions at temperatures near the cloud point.}, subject = {Weiche Materie}, language = {en} } @phdthesis{Hetterich2018, author = {Hetterich, Daniel Marcus}, title = {Localization within disordered systems of star-like topology}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-169318}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {This Thesis investigates the interplay of a central degree of freedom with an environment. Thereby, the environment is prepared in a localized phase of matter. The long-term aim of this setup is to store quantum information on the central degree of freedom while exploiting the advantages of localized systems. These many-body localized systems fail to equilibrate under the description of thermodynamics, mostly due to disorder. Doing so, they form the most prominent phase of matter that violates the eigenstate thermalization hypothesis. Thus, many-body localized systems preserve information about an initial state until infinite times without the necessity to isolate the system. This unique feature clearly suggests to store quantum information within localized environments, whenever isolation is impracticable. After an introduction to the relevant concepts, this Thesis examines to which extent a localized phase of matter may exist at all if a central degree of freedom dismantles the notion of locality in the first place. To this end, a central spin is coupled to the disordered Heisenberg spin chain, which shows many-body localization. Furthermore, a noninteracting analog describing free fermions is discussed. Therein, an impurity is coupled to an Anderson localized environment. It is found that in both cases, the presence of the central degree of freedom manifests in many properties of the localized environment. However, for a sufficiently weak coupling, quantum chaos, and thus, thermalization is absent. In fact, it is shown that the critical disorder, at which the metal-insulator transition of its environment occurs in the absence of the central degree of freedom, is modified by the coupling strength of the central degree of freedom. To demonstrate this, a phase diagram is derived. Within the localized phase, logarithmic growth of entanglement entropy, a typical signature of many-body localized systems, is increased by the coupling to the central spin. This property is traced back to resonantly coupling spins within the localized Heisenberg chain and analytically derived in the absence of interactions. Thus, the studied model of free fermions is the first model without interactions that mimics the logarithmic spreading of entanglement entropy known from many-body localized systems. Eventually, it is demonstrated that observables regarding the central spin significantly break the eigenstate thermalization hypothesis within the localized phase. Therefore, it is demonstrated how a central spin can be employed as a detector of many-body localization.}, subject = {Quanteninformatik}, language = {en} } @article{JahnkeGiesAssmannetal.2016, author = {Jahnke, Frank and Gies, Christopher and Aßmann, Marc and Bayer, Manfred and Leymann, H.A.M. and Foerster, Alexander and Wiersig, Jan and Schneider, Christian and Kamp, Martin and H{\"o}fling, Sven}, title = {Giant photon bunching, superradiant pulse emission and excitation trapping in quantum-dot nanolasers}, series = {Nature Communications}, volume = {7}, journal = {Nature Communications}, number = {11540}, doi = {10.1038/ncomms11540}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166144}, year = {2016}, abstract = {Light is often characterized only by its classical properties, like intensity or coherence. When looking at its quantum properties, described by photon correlations, new information about the state of the matter generating the radiation can be revealed. In particular the difference between independent and entangled emitters, which is at the heart of quantum mechanics, can be made visible in the photon statistics of the emitted light. The well-studied phenomenon of superradiance occurs when quantum-mechanical correlations between the emitters are present. Notwithstanding, superradiance was previously demonstrated only in terms of classical light properties. Here, we provide the missing link between quantum correlations of the active material and photon correlations in the emitted radiation. We use the superradiance of quantum dots in a cavity-quantum electrodynamics laser to show a direct connection between superradiant pulse emission and distinctive changes in the photon correlation function. This directly demonstrates the importance of quantum-mechanical correlations and their transfer between carriers and photons in novel optoelectronic devices.}, language = {en} } @phdthesis{Lutz2018, author = {Lutz, Peter}, title = {Surface and Interface Electronic Structure in Ferroelectric BaTiO\(_3\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-159057}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Transition metal oxides (TMO) represent a highly interesting material class as they exhibit a variety of different emergent phenomena including multiferroicity and superconductivity. These effects result from a significant interplay of charge, spin and orbital degrees of freedom within the correlated d-electrons. Oxygen vacancies (OV) at the surface of certain d0 TMO release free charge carriers and prompt the formation of a two-dimensional electron gas (2DEG). Barium titanate (BaTiO3) is a prototypical and promising d0 TMO. It displays ferroelectricity at room temperature and features several structural phase transitions, from cubic over tetragonal (at room temperature) and orthorhombic to rhombohedral. The spontaneous electric polarization in BaTiO3 can be used to manipulate the physical properties of adjacent materials, e.g. in thin films. Although the macroscopic properties of BaTiO3 are studied in great detail, the microscopic electronic structure at the surface and interface of BaTiO3 is not sufficiently understood yet due to the complex interplay of correlation within the d states, oxygen vacancies at the surface, ferroelectricity in the bulk and the structural phase transitions in BaTiO3. This thesis investigates the electronic structure of different BaTiO3 systems by means of angle-resolved photoelectron spectroscopy (ARPES). The valence band of BaTiO3 single crystals is systematically characterized and compared to theoretical band structure calculations. A finite p-d hybridization of titanium and oxygen states was inferred at the high binding energy side of the valence band. In BaTiO3 thin films, the occurrence of spectral weight near the Fermi level could be linked to a certain amount of OV at the surface which effectively dopes the host system. By a systematic study of the metallic surface states as a function of temperature and partial oxygen pressure, a model was established which reflects the depletion and accumulation of charge carriers at the surface of BaTiO3. An instability at T ~ 285K assumes a volatile behavior of these surface states. The ferroelectricity in BaTiO3 allows a control of the electronic structure at the interface of BaTiO3-based heterostructures. Therefore, the interface electronic structure of Bi/BaTiO3 was studied with respect to the strongly spin-orit coupled states in Bi by also including a thickness dependent characterization. The ARPES results, indeed, confirm the presence of Rashba spin-split electronic states in the bulk band gap of the ferroelectric substrate. By varying the film thickness in Bi/BaTiO3, it was able to modify the energy position and the Fermi vector of the spin-split states. This observation is associated with the appearance of an interface state which was observed for very low film thickness. Both spectral findings suggest a significant coupling between the Bi films and BaTiO3.}, subject = {Bariumtitanat}, language = {en} } @article{OPUS4-22085, title = {Measurement of inclusive jet and dijet cross-sections in proton-proton collisions at root s=13 TeV with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {195}, journal = {Journal of High Energy Physics}, number = {5}, organization = {The ATLAS collaboration}, doi = {10.1007/JHEP05(2018)195}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220853}, pages = {1-48}, year = {2018}, abstract = {Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The measurement uses a dataset with an integrated luminosity of 3.2 fb(-1) recorded in 2015 with the ATLAS detector at the Large Hadron Collider. Jets are identified using the anti-lit algorithm with a radius parameter value of R = 0.4. The inclusive jet cross-sections are measured double-differentially as a function of the jet transverse momentum, covering the range from 100 GeV to 3.5 TeV, and the absolute jet rapidity up to vertical bar y vertical bar = 3. The double-differential dijet production cross-sections are presented as a function of the dijet mass, covering the range from 300 GeV to 9 TeV, and the half absolute rapidity separation between the two leading jets within vertical bar y vertical bar < 3, y*, up to y* = 3. Next-to-leading-order, and next-to-next-to-leading-order for the inclusive jet measurement, perturbative QCD calculations corrected for non-perturbative and electroweak effects are compared to the measured cross-sections.}, language = {en} } @article{OPUS4-16515, title = {The prototype detection unit of the KM3NeT detector}, series = {The European Physical Journal C}, volume = {76}, journal = {The European Physical Journal C}, number = {54}, organization = {KM3NeT Collaboration}, doi = {10.1140/epjc/s10052-015-3868-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-165159}, year = {2016}, abstract = {A prototype detection unit of the KM3NeT deep-sea neutrino telescope has been installed at 3500m depth 80 km offshore the Italian coast. KM3NeT in its final configuration will contain several hundreds of detection units. Each detection unit is a mechanical structure anchored to the sea floor, held vertical by a submerged buoy and supporting optical modules for the detection of Cherenkov light emitted by charged secondary particles emerging from neutrino interactions. This prototype string implements three optical modules with 31 photomultiplier tubes each. These optical modules were developed by the KM3NeT Collaboration to enhance the detection capability of neutrino interactions. The prototype detection unit was operated since its deployment in May 2014 until its decommissioning in July 2015. Reconstruction of the particle trajectories from the data requires a nanosecond accuracy in the time calibration. A procedure for relative time calibration of the photomultiplier tubes contained in each optical module is described. This procedure is based on the measured coincidences produced in the sea by the 40K background light and can easily be expanded to a detector with several thousands of optical modules. The time offsets between the different optical modules are obtained using LED nanobeacons mounted inside them. A set of data corresponding to 600 h of livetime was analysed. The results show good agreement with Monte Carlo simulations of the expected optical background and the signal from atmospheric muons. An almost background-free sample of muons was selected by filtering the time correlated signals on all the three optical modules. The zenith angle of the selected muons was reconstructed with a precision of about 3∘.}, language = {en} } @article{LindertPozzoriniBoughezaletal.2017, author = {Lindert, J. M. and Pozzorini, S. and Boughezal, R. and Campbell, J. M. and Denner, A. and Dittmaier, S. and Gehrmann-De Ridder, A. and Gehrmann, T. and Glover, N. and Huss, A. and Kallweit, S. and Maierh{\"o}fer, P. and Mangano, M. L. and Morgan, T. A. and M{\"u}ck, A. and Petriello, F. and Salam, G. P. and Sch{\"o}nherr, M. and Williams, C.}, title = {Precise predictions for \(V+\)jets dark matter backgrounds}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, doi = {10.1140/epjc/s10052-017-5389-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172555}, year = {2017}, abstract = {High-energy jets recoiling against missing transverse energy (MET) are powerful probes of dark matter at the LHC. Searches based on large MET signatures require a precise control of the \({Z(ν\overline{ν})}+\) jet background in the signal region. This can be achieved by taking accurate data in control regions dominated by \(Z(ℓ^+ℓ^-)+\) jet, \(W(ℓν)+\) jet and \(γ+\) jet production, and extrapolating to the \({Z(ν\overline{ν})}+\) jet background by means of precise theoretical predictions. In this context, recent advances in perturbative calculations open the door to significant sensitivity improvements in dark matter searches. In this spirit, we present a combination of state-of-the-art calculations for all relevant \(V+\) jets processes, including throughout NNLO QCD corrections and NLO electroweak corrections supplemented by Sudakov logarithms at two loops. Predictions at parton level are provided together with detailed recommendations for their usage in experimental analyses based on the reweighting of Monte Carlo samples. Particular attention is devoted to the estimate of theoretical uncertainties in the framework of dark matter searches, where subtle aspects such as correlations across different \(V+\) jet processes play a key role. The anticipated theoretical uncertainty in the \({Z(ν\overline{ν})}+\) jet background is at the few percent level up to the TeV range.}, language = {en} } @article{BiedermannBilloniDenneretal.2016, author = {Biedermann, B. and Billoni, M. and Denner, A. and Dittmaier, S. and Hofer, L. and J{\"a}ger, B. and Salfelder, L.}, title = {Next-to-leading-order electroweak corrections to pp -> W\(^{+}\)W\(^{-}\) -> 4 leptons at the LHC}, series = {JOURNAL OF HIGH ENERGY PHYSICS}, volume = {06}, journal = {JOURNAL OF HIGH ENERGY PHYSICS}, number = {065}, doi = {10.1007/JHEP06(2016)065}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-167790}, year = {2016}, abstract = {We present results of the first calculation of next-to-leading-order electroweak corrections to W-boson pair production at the LHC that fully takes into account leptonic W-boson decays and off-shell effects. Employing realistic event selections, we discuss the corrections in situations that are typical for the study of W-boson pairs as a signal process or of Higgs-boson decays H → WW∗, to which W-boson pair production represents an irreducible background. In particular, we compare the full off-shell results, obtained treating the W-boson resonances in the complex-mass scheme, to previous results in the so-called double-pole approximation, which is based on an expansion of the loop amplitudes about the W resonance poles. At small and intermediate scales, i.e. in particular in angular and rapidity distributions, the two approaches show the expected agreement at the level of fractions of a percent, but larger differences appear in the TeV range. For transverse-momentum distributions, the differences can even exceed the 10\% level in the TeV range where "background diagrams" with one instead of two resonant W bosons gain in importance because of recoil effects.}, language = {en} } @article{MunzJakobBorisjuk2016, author = {Munz, Eberhard and Jakob, Peter M. and Borisjuk, Ljudmilla}, title = {The potential of nuclear magnetic resonance to track lipids in planta}, series = {Biochimie}, volume = {130}, journal = {Biochimie}, doi = {10.1016/j.biochi.2016.07.014}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-186828}, pages = {97-108}, year = {2016}, abstract = {Nuclear Magnetic Resonance (NMR) provides a highly flexible platform for non invasive analysis and imaging biological samples, since the manipulation of nuclear spin allows the tailoring of experiments to maximize the informativeness of the data. MRI is capable of visualizing a holistic picture of the lipid storage in living plant/seed. This review has sought to explain how the technology can be used to acquire functional and physiological data from plant samples, and how to exploit it to characterize lipid deposition in vivo. At the same time, we have referred to the current limitations of NMR technology as applied to plants, and in particular of the difficulty of transferring methodologies optimized for animal/medical subjects to plant ones. A forward look into likely developments in the field is included, anticipating its key future role in the study of living plant.}, language = {en} } @article{OPUS4-17217, title = {All-sky search for high-energy neutrinos from gravitational wave event GW170104 with the ANTARES neutrino telescope}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, organization = {The ANTARES Collaboration}, doi = {10.1140/epjc/s10052-017-5451-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172174}, year = {2017}, abstract = {Advanced LIGO detected a significant gravitational wave signal (GW170104) originating from the coalescence of two black holes during the second observation run on January 4th, 2017. An all-sky high-energy neutrino follow-up search has been made using data from the Antares neutrino telescope, including both upgoing and downgoing events in two separate analyses. No neutrino candidates were found within ±500 s around the GW event time nor any time clustering of events over an extended time window of ±3 months. The non-detection is used to constrain isotropic-equivalent high-energy neutrino emission from GW170104 to less than ∼ 1.2 × \(10^{55}\) erg for a \(E^{-2}\) spectrum. This constraint is valid in the energy range corresponding to the 5-95\% quantiles of the neutrino flux [3.2 TeV; 3.6 PeV], if the GW emitter was below the Antares horizon at the alert time.}, language = {en} } @article{RothmayrGuarinCastroHartmannetal.2022, author = {Rothmayr, Florian and Guarin Castro, Edgar David and Hartmann, Fabian and Knebl, Georg and Schade, Anne and H{\"o}fling, Sven and Koeth, Johannes and Pfenning, Andreas and Worschech, Lukas and Lopez-Richard, Victor}, title = {Resonant tunneling diodes: mid-infrared sensing at room temperature}, series = {Nanomaterials}, volume = {12}, journal = {Nanomaterials}, number = {6}, issn = {2079-4991}, doi = {10.3390/nano12061024}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-267152}, year = {2022}, abstract = {Resonant tunneling diode photodetectors appear to be promising architectures with a simple design for mid-infrared sensing operations at room temperature. We fabricated resonant tunneling devices with GaInAsSb absorbers that allow operation in the 2-4 μm range with significant electrical responsivity of 0.97 A/W at 2004 nm to optical readout. This paper characterizes the photosensor response contrasting different operational regimes and offering a comprehensive theoretical analysis of the main physical ingredients that rule the sensor functionalities and affect its performance. We demonstrate how the drift, accumulation, and escape efficiencies of photogenerated carriers influence the electrostatic modulation of the sensor's electrical response and how they allow controlling the device's sensing abilities.}, language = {en} } @article{GabelPickemScheidereretal.2022, author = {Gabel, Judith and Pickem, Matthias and Scheiderer, Philipp and Dudy, Lenart and Leikert, Berengar and Fuchs, Marius and St{\"u}binger, Martin and Schmitt, Matthias and K{\"u}spert, Julia and Sangiovanni, Giorgio and Tomczak, Jan M. and Held, Karsten and Lee, Tien-Lin and Claessen, Ralph and Sing, Michael}, title = {Toward Functionalized Ultrathin Oxide Films: The Impact of Surface Apical Oxygen}, series = {Advanced Electronic Materials}, volume = {8}, journal = {Advanced Electronic Materials}, number = {4}, issn = {2199-160X}, doi = {10.1002/aelm.202101006}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-318914}, year = {2022}, abstract = {Thin films of transition metal oxides open up a gateway to nanoscale electronic devices beyond silicon characterized by novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO\(_{3}\) as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen or—mutatis mutandis—the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides near the single unit-cell limit.}, language = {en} } @article{FrankPflaum2022, author = {Frank, Maximilian and Pflaum, Jens}, title = {Tuning Electronic and Ionic Transport by Carbon-Based Additives in Polymer Electrolytes for Thermoelectric Applications}, series = {Advanced Functional Materials}, volume = {32}, journal = {Advanced Functional Materials}, number = {32}, issn = {1616-301X}, doi = {10.1002/adfm.202203277}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-318908}, year = {2022}, abstract = {Thermoelectric materials utilizing ionic transport open-up entirely new possibilities for the recuperation of waste heat. Remarkably, solid state electrolytes which have entered the focus of battery research in recent years turn-out to be promising candidates also for ionic thermoelectrics. Here, the dynamics of ionic transport and thermoelectric properties of a methacrylate based polymer blend in combination with a lithium salt is analyzed. Impedance spectroscopy data indicates the presence of just one transport mechanism irrespective of lithium salt concentration. In contrast, the temperature dependent ionic conductivity increases with salt concentration and can be ascribed to a Vogel-Fulcher-Tammann (VFT) behavior. The obtained Seebeck coefficients of 2 mV K\(^{-1}\) allow for high power outputs while the polymer matrix maintains the temperature gradient by its low thermal conductivity. Adding multi-walled carbon nanotubes to the polymer matrix allows for variation of the Seebeck coefficient as well as the ionic and electronic conductivities. As a result, a transition between a high temperature VFT regime and a low temperature Arrhenius regime appears at a critical temperature, T\(_{c}\), shifting upon addition of salt. The observed polarity change in Seebeck voltage at T\(_{c}\) suggests a new mode of thermoelectric operation, which is demonstrated by a proof-of-concept mixed electronic-ionic-thermoelectric generator.}, language = {en} } @article{OPUS4-22546, title = {A search for pair-produced resonances in four-jet final states at root s=13 TeV with the ATLAS detector}, series = {The European Physical Journal C}, volume = {78}, journal = {The European Physical Journal C}, number = {250}, organization = {The ATLAS Collaboration}, doi = {10.1140/epjc/s10052-018-5693-4}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-225465}, pages = {1-28}, year = {2018}, abstract = {A search for massive coloured resonances which are pair-produced and decay into two jets is presented. The analysis uses 36.7 fb(-1) of root s = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the background prediction is observed. Results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is the top squark, (t) over tilde, which decays promptly into two quarks through R-parity-violating couplings. Top squarks with masses in the range 100 GeV < m((T) over tilde) < 410 GeV are excluded at 95\% confidence level. If the decay is into a b-quark and a light quark, a dedicated selection requiring two b-tags is used to exclude masses in the ranges 100 GeV < m((t) over tilde) < 470 GeV and 480 GeV < m(<(t)over tilde>) < 610 GeV. Additional limits are set on the pair-production of massive colour-octet resonances.}, language = {en} } @article{OPUS4-22086, title = {Search for a new heavy gauge-boson resonance decaying into a lepton and missing transverse momentum in 36 fb\(^{-1}\) of \({pp}\) collisions at root s=13 TeV with the ATLAS experiment}, series = {The European Physical Journal C}, volume = {78}, journal = {The European Physical Journal C}, number = {401}, organization = {The ATLAS Collaboration}, doi = {10.1140/epjc/s10052-018-5877-y}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220869}, pages = {1-23}, year = {2018}, abstract = {The results of a search for new heavy W' bosons decaying to an electron or muon and a neutrino using proton-proton collision data at a centre-of-mass energy of root s = 13 TeV are presented. The dataset was collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb(-1). As no excess of events above the Standard Model prediction is observed, the results are used to set upper limits on the W' boson cross-section times branching ratio to an electron or muon and a neutrino as a function of the W' mass. Assuming a W' boson with the same couplings as the Standard Model W boson, W' masses below 5.1 TeV are excluded at the 95\% confidence level.}, language = {en} } @article{OPUS4-22670, title = {Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at root s=13 TeV with the ATLAS detector}, series = {Physical Review D}, volume = {97}, journal = {Physical Review D}, number = {5}, organization = {The ATLAS Collaboration}, doi = {10.1103/PhysRevD.97.052010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226707}, pages = {1-35}, year = {2018}, abstract = {A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 tb(-1) of integrated luminosity at root s = 13 TeV. Events with same flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95\% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.}, language = {en} } @article{OPUS4-22082, title = {Measurement of the inclusive and fiducial t(t)over-bar production cross-sections in the lepton+jets channel in \({pp}\) collisions at root s=8 TeV with the ATLAS detector}, series = {The European Physical Journal C}, volume = {78}, journal = {The European Physical Journal C}, number = {487}, organization = {The ATLAS Collaboration}, doi = {10.1140/epjc/s10052-018-5904-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220827}, pages = {1-31}, year = {2018}, abstract = {The inclusive and fiducial t (t) over bar production cross sections are measured in the lepton+jets channel using 20.2 fb(-1) of proton proton collision data at a centre-of mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W+jets process is modelled using Z+jets events in a data-driven approach. The inclusive t (t) over bar cross-section is measured with a precision of 5.7\% to be (sigma(inc) (t (t) over bar) = 248.3 +/- 0.7 (stat.) +/- 13.4 (syst.) +/- 4.7 (lumi.) pb, assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is sigma(fid) (t (t) over bar) = 48.8 +/- 0.1 (stat.) +/- 2.0 (syst.) +/- 0.9 (lumi.) pb with a precision of 4.5\%.}, language = {en} } @article{OPUS4-22613, title = {Search for pair production of up-type vector-like quarks and for four-top-quark events in final states with multiple \(b\)-jets with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {89}, journal = {Journal of High Energy Physics}, number = {7}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP07(2018)089}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226131}, pages = {1-67}, year = {2018}, abstract = {A search for pair production of up-type vector-like quarks (T) with a significant branching ratio into a top quark and either a Standard Model Higgs boson or a Z boson is presented. The same analysis is also used to search for four-top-quark production in several new physics scenarios. The search is based on a dataset of pp collisions at root s = 13TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb(-1). Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets, as well as the jets+E-T(miss) final state, characterised by multiple jets and large missing transverse momentum. The search exploits the high multiplicity of jets identified as originating from b-quarks, and the presence of boosted, hadronically decaying top quarks and Higgs bosons reconstructed as large-radius jets, characteristic of signal events. No significant excess above the Standard Model expectation is observed, and 95\% CL upper limits are set on the production cross sections for the different signal processes considered. These cross-section limits are used to derive lower limits on the mass of a vector-like T quark under several branching ratio hypotheses assuming contributions from T -> Wb, Zt, Ht decays. The 95\% CL observed lower limits on the T quark mass range between 0.99TeV and 1.43TeV for all possible values of the branching ratios into the three decay modes considered, significantly extending the reach beyond that of previous searches. Additionally, upper limits on anomalous four-top-quark production are set in the context of an effective field theory model, as well as in an universal extra dimensions model.}, language = {en} } @article{OPUS4-31736, title = {Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in \({pp}\) collisions at root s=13 TeV with the ATLAS detector}, series = {Physical Review D}, volume = {99}, journal = {Physical Review D}, number = {9}, organization = {The ATLAS Collaboration}, doi = {10.1103/PhysRevD.99.092004}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-317362}, pages = {1-38}, year = {2019}, abstract = {A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of root s = 13 TeV corresponding to an integrated luminosity of 36.1 fb(-1). Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z' bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the Z' boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1-3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.}, language = {en} } @article{OPUS4-22600, title = {Search for pair production of Higgs bosons in the \({bb̅bb̅}\) final state using proton-proton collisions at root s=13 TeV with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {30}, journal = {Journal of High Energy Physics}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP01(2019)030}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226001}, pages = {1-48}, year = {2018}, abstract = {A search for Higgs boson pair production in the bbbb final state is carried out with up to 36.1 fb(-1) of LHC proton-proton collision data collected at s=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260-3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bbbb are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95\% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.}, language = {en} } @phdthesis{Scheffler2023, author = {Scheffler, Lukas}, title = {Molecular beam epitaxy of the half-Heusler antiferromagnet CuMnSb}, doi = {10.25972/OPUS-32283}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322839}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {This work presents a newly developed method for the epitaxial growth of the half-Heusler antiferromagnet CuMnSb. All necessary process steps, from buffer growth to the deposition of a protective layer, are presented in detail. Using structural, electrical, and magnetic characterization, the material parameters of the epitaxial CuMnSb layers are investigated. The successful growth of CuMnSb by molecular beam epitaxy is demonstrated on InAs (001), GaSb (001), and InP (001) substrates. While CuMnSb can be grown pseudomorphically on InAs and GaSb, the significant lattice mismatch for growth on InP leads to relaxation already at low film thicknesses. Due to the lower conductivity of GaSb compared to InAs, GaSb substrates are particularly suitable for the fabrication of CuMnSb layers for lateral electrical transport experiments. However, by growing a high-resistive ZnTe interlayer below the CuMnSb layer, lateral transport experiments on CuMnSb layers grown on InAs can also be realized. Protective layers of Ru and Al2O3 have proven to be suitable for protecting the CuMnSb layers from the environment. Structural characterization by high resolution X-ray diffraction (full width at half maximum of 7.7 ′′ of the rocking curve) and atomic force microscopy (root mean square surface roughness of 0.14 nm) reveals an outstanding crystal quality of the epitaxial CuMnSb layers. The half-Heusler crystal structure is confirmed by scanning transmission electron microscopy and the stoichiometric material composition by Rutherford backscattering spectrometry. In line with the high crystal quality, a new minimum value of the residual resistance of CuMnSb (𝜌0 = 35 μΩ ⋅ cm) could be measured utilizing basic electrical transport experiments. An elaborate study of epitaxial CuMnSb grown on GaSb reveals a dependence of the vertical lattice parameter on the Mn/Sb flux ratio. This characteristic enables the growth of tensile, unstrained, and compressive strained CuMnSb layers on a single substrate material. Additionally, it is shown that the N{\´e}el temperature has a maximum of 62 K at stoichiometric material composition and thus can be utilized as a selection tool for stoichiometric CuMnSb samples. Mn-related defects are believed to be the driving force for these observations. The magnetic characterization of the epitaxial CuMnSb films is performed by superconducting quantum interference device magnetometry. Magnetic behavior comparable to the bulk material is found, however, an additional complex magnetic phase appears in thin CuMnSb films and/or at low magnetic fields, which has not been previously reported for CuMnSb. This magnetic phase is believed to be localized at the CuMnSb surface and exhibits both superparamagnetic and spin-glass-like behavior. The exchange bias effect of CuMnSb is investigated in combination with different in- and out-of-plane ferromagnets. It is shown that the exchange bias effect can only be observed in combination with in-plane ferromagnets. Finally, the first attempts at the growth of fully epitaxial CuMnSb/NiMnSb heterostructures are presented. Both magnetic and structural studies by secondary-ion mass spectrometry indicate the interdiffusion of Cu and Ni atoms between the two half-Heusler layers, however, an exchange bias effect can be observed for the CuMnSb/NiMnSb heterostructures. Whether this exchange bias effect originates from exchange interaction between the CuMnSb and NiMnSb layers, or from ferromagnetic inclusions in the antiferromagnetic layer can not be conclusively identified.}, subject = {Molekularstrahlepitaxie}, language = {en} } @article{OPUS4-22069, title = {Measurement of the Soft-Drop Jet Mass in \({pp}\) Collisions at root s=13 TeV with the ATLAS Detector}, series = {Physical Review Letters}, volume = {121}, journal = {Physical Review Letters}, number = {9}, organization = {The ATLAS Collaboration}, doi = {10.1103/PhysRevLett.121.092001}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220694}, pages = {1-20}, year = {2018}, abstract = {Jet substructure observables have significantly extended the search program for physics beyond the standard model at the Large Hadron Collider. The state-of-the-art tools have been motivated by theoretical calculations, but there has never been a direct comparison between data and calculations of jet substructure observables that are accurate beyond leading-logarithm approximation. Such observables are significant not only for probing the collinear regime of QCD that is largely unexplored at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. This Letter documents a measurement of the first jet substructure quantity at a hadron collider to be calculated at next-to-next-to-leading-logarithm accuracy. The normalized, differential cross section is measured as a function of log(10)rho(2), where rho is the ratio of the soft-drop mass to the ungroomed jet transverse momentum. This quantity is measured in dijet events from 32.9 fb(-1) of root s = 13 TeV proton-proton collisions recorded by the ATLAS detector. The data are unfolded to correct for detector effects and compared to precise QCD calculations and leading-logarithm particle-level Monte Carlo simulations.}, language = {en} } @article{OPUS4-22593, title = {Search for dark matter produced in association with bottom or top quarks in root s=13 TeV \({pp}\) collisions with the ATLAS detector}, series = {European Physical Journal C}, volume = {C 78}, journal = {European Physical Journal C}, number = {18}, organization = {The ATLAS Collaboration}, doi = {10.1140/epjc/s10052-017-5486-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-225937}, pages = {1-36}, year = {2018}, abstract = {A search for weakly interacting massive dark matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1 fb(-1) of proton proton collision data recorded by the ATLAS experiment at root s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements.}, language = {en} } @article{OPUS4-22602, title = {Measurements of differential cross sections of top quark pair production in association with jets in pp collisions at root s=13 TeV using the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {159}, journal = {Journal of High Energy Physics}, number = {10}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP10(2018)159}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226023}, pages = {1-57}, year = {2018}, abstract = {Measurements of di ff erential cross sections of top quark pair production in association with jets by the ATLAS experiment at the LHC are presented. The measurements are performed as functions of the top quark transverse momentum, the transverse momentum of the top quark-antitop quark system and the out-of-plane transverse momentum using data from pp collisions at p s = 13TeV collected by the ATLAS detector at the LHC in 2015 and corresponding to an integrated luminosity of 3.2 fb. The top quark pair events are selected in the lepton (electron or muon) + jets channel. The measured cross sections, which are compared to several predictions, allow a detailed study of top quark production.}, language = {en} } @article{OPUS4-22611, title = {Search for flavour-changing neutral current top-quark decays \(t\) -> \({qZ}\) in proton-proton collisions at root s=13 TeV with the ATLAS detector}, series = {Journal of High Energy Physics}, volume = {176}, journal = {Journal of High Energy Physics}, number = {7}, organization = {The ATLAS Collaboration}, doi = {10.1007/JHEP07(2018)176}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226114}, pages = {1-40}, year = {2018}, abstract = {A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of root s = 13TeV, corresponding to an integrated luminosity of 36.1 fb(-1), are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t -> qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t -> bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95\% confidence level the search sets observed (expected) upper limits of 1.7 x 10(-4) (2.4 x 10(-4)) on the t -> uZ branching ratio and 2.4 x 10(-4) (3.2 x 10(-4)) on the t -> cZ branching ratio, constituting the most stringent limits to date.}, language = {en} } @article{OPUS4-22615, title = {Measurement of the production cross section of three isolated photons in \({pp}\) collisions at root \(s\)=8 TeV using the ATLAS detector}, series = {Physics Letters B}, volume = {781}, journal = {Physics Letters B}, organization = {The ATLAS Collaboration}, doi = {10.1016/j.physletb.2018.03.057}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226154}, pages = {55-76}, year = {2018}, abstract = {A measurement of the production of three isolated photons in proton-proton collisions at a centre-of-mass energy root s = 8 TeV is reported. The results are based on an integrated luminosity of 20.2 fb(-1) collected with the ATLAS detector at the LHC. The differential cross sections are measured as functions of the transverse energy of each photon, the difference in azimuthal angle and in pseudorapidity between pairs of photons, the invariant mass of pairs of photons, and the invariant mass of the triphoton system. A measurement of the inclusive fiducial cross section is also reported. Next-to-leading-order perturbative QCD predictions are compared to the cross-section measurements. The predictions underestimate the measurement of the inclusive fiducial cross section and the differential measurements at low photon transverse energies and invariant masses. They provide adequate descriptions of the measurements at high values of the photon transverse energies, invariant mass of pairs of photons, and invariant mass of the triphoton system. (C) 2018 The Author. Published by Elsevier B.V.}, language = {en} } @article{GlinzŠleichrtKytyřetal.2021, author = {Glinz, Jonathan and Šleichrt, Jan and Kyt{\´y}ř, Daniel and Ayalur-Karunakaran, Santhosh and Zabler, Simon and Kastner, Johann and Senck, Sascha}, title = {Phase-contrast and dark-field imaging for the inspection of resin-rich areas and fiber orientation in non-crimp vacuum infusion carbon-fiber-reinforced polymers}, series = {Journal of Materials Science}, volume = {56}, journal = {Journal of Materials Science}, number = {16}, doi = {10.1007/s10853-021-05907-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-351581}, pages = {9712-9727}, year = {2021}, abstract = {In this work, we present a multimodal approach to three-dimensionally quantify and visualize fiber orientation and resin-rich areas in carbon-fiber-reinforced polymers manufactured by vacuum infusion. Three complementary image modalities were acquired by Talbot-Lau grating interferometer (TLGI) X-ray microcomputed tomography (XCT). Compared to absorption contrast (AC), TLGI-XCT provides enhanced contrast between polymer matrix and carbon fibers at lower spatial resolutions in the form of differential phase contrast (DPC) and dark-field contrast (DFC). Consequently, relatively thin layers of resin, effectively indiscernible from image noise in AC data, are distinguishable. In addition to the assessment of fiber orientation, the combination of DPC and DFC facilitates the quantification of resin-rich areas, e.g., in gaps between fiber layers or at binder yarn collimation sites. We found that resin-rich areas between fiber layers are predominantly developed in regions characterized by a pronounced curvature. In contrast, in-layer resin-rich areas are mainly caused by the collimation of fibers by binder yarn. Furthermore, void volume around two adjacent 90°-oriented fiber layers is increased by roughly 20\% compared to a random distribution over the whole specimen.}, language = {en} } @phdthesis{Miller2024, author = {Miller, Kirill}, title = {Untersuchung von Nanostrukturen basierend auf LaAlO\(_3\)/SrTiO\(_3\) f{\"u}r Anwendungen in nicht von-Neumann-Rechnerarchitekturen}, doi = {10.25972/OPUS-35472}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-354724}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {Die Dissertation besch{\"a}ftigt sich mit der Analyse von oxidischen Nanostrukturen. Die Grundlage der Bauelemente stellt dabei die LaAlO3/SrTiO3-Heterostruktur dar. Hierbei entsteht an der Grenzfl{\"a}che beider {\"U}bergangsmetalloxide ein quasi zweidimensionales Elektronengas, welches wiederum eine F{\"u}lle von beachtlichen Eigenschaften und Charakteristika zeigt. Mithilfe lithographischer Verfahren wurden zwei unterschiedliche Bauelemente verwirklicht. Dabei handelt es sich einerseits um einen planaren Nanodraht mit lateralen Gates, welcher auf der Probenoberfl{\"a}che prozessiert wurde und eine bemerkenswerte Trialit{\"a}t aufweist. Dieses Bauelement kann unter anderem als ein herk{\"o}mmlicher Feldeffekttransistor agieren, wobei der Ladungstransport durch die lateral angelegte Spannung manipuliert wird. Zus{\"a}tzlich konnten auch Speichereigenschaften beobachtet werden, sodass das gesamte Bauelement als ein sogenannter Memristor fungieren kann. In diesem Fall h{\"a}ngt der Ladungstransport von der Elektronenakkumulation auf den lateralen potentialfreien Gates ab. Die Memristanz des Nanodrahts l{\"a}sst sich unter anderem durch Lichtleistungen im Nanowattbereich und mithilfe von kurzen Spannungspulsen ver{\"a}ndern. Dar{\"u}ber hinaus kann die Elektronenakkumulation auch in Form einer memkapazitiven Charakteristik beobachtet werden. Neben dem Nanodraht wurde auch eine Kreuzstruktur, die eine erg{\"a}nzende ferromagnetischen Elektrode beinhaltet, realisiert. Mit diesem neuartigen Bauteil wird die Umwandlung zwischen Spin- und Ladungsstr{\"o}men innerhalb der nanoskaligen Struktur untersucht. Hierbei wird die starke Spin-Bahn-Kopplung im quasi zweidimensionalen Elektronengas ausgenutzt.}, subject = {Memristor}, language = {de} } @article{CernaVelazcoFaberJonesPerezetal.2017, author = {Cerna-Velazco, Nhell and Faber, Thomas and Jones-P{\´e}rez, Joel and Porod, Werner}, title = {Constraining sleptons at the LHC in a supersymmetric low-scale seesaw scenario}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, doi = {10.1140/epjc/s10052-017-5231-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173809}, year = {2017}, abstract = {We consider a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. We extend the Minimal Supersymmetric Standard Model by adding light right-handed neutrinos and their superpartners, the R-sneutrinos, and consider the lightest neutralinos to be higgsino-like. We consider the possibilities of having either an R-sneutrino or a higgsino as lightest supersymmetric particle. Assuming that squarks and gauginos are heavy, we systematically evaluate the bounds on slepton masses due to existing LHC data.}, language = {en} } @phdthesis{Mueller2022, author = {M{\"u}ller, Valentin Leander}, title = {Transport signatures of topological and trivial states in the three-dimensional topological insulator HgTe}, doi = {10.25972/OPUS-25952}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-259521}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {The thesis at hand is concerned with improving our understanding of and our control over transport properties of the three-dimensional topological insulator HgTe. Topological insulators are characterized by an insulating bulk and symmetry-protected metallic surface states. These topological surface states hold great promise for research and technology; at the same time, many properties of experimentally accessible topological insulator materials still need to be explored thoroughly. The overall aim of this thesis was to experimentally investigate micrometer-sized HgTe transport devices to observe the ballistic transport regime as well as intercarrier scattering and possibly identify special properties of the topological surface states. Part I of the thesis presents lithographic developments concerned with etching small HgTe devices. The aim was to replace existing processes which relied on dry etching with high-energy \(\text{Ar}^+\) ions and an organic etch mask. This etching method is known to degrade the HgTe crystal quality. In addition, the etch mask turned out to be not durable for long etching processes and difficult to remove completely after etching. First, \(\text{BaF}_2\) was introduced as a new etch mask for dry etching to replace the organic etch mask. With common surface characterization techniques like SEM and XPS it was shown that \(\text{BaF}_2\) etch masks are easy to deposit, highly durable in common dry etching processes for \(\text{Hg}_{1-x}\text{Cd}_x\text{Te}\), and easy to remove in deionized water. Transport results of HgTe devices fabricated with the new etch mask are comparable to results obtained with the old process. At the same time, the new etch mask can withstand longer etching times and does not cause problems due to incomplete removal. Second, a new inductively coupled plasma dry etching process based on \(\text{CH}_4\) and Ar was introduced. This etching process is compatible with \(\text{BaF}_2\) etch masks and yields highly reproducible results. Transport results indicate that the new etching process does not degrade the crystal quality and is suitable to produce high-quality transport devices even in the micrometer range. A comparison with wet-etched samples shows that inductively coupled plasma etching introduces a pronounced edge roughness. This - usually undesirable - property is actually beneficial for some of the experiments in this study and mostly irrelevant for others. Therefore, most samples appearing in this thesis were fabricated with the new process. Part II of the thesis details the advancements made in identifying topological and trivial states which contribute to transport in HgTe three-dimensional topological insulators. To this end, macroscopic Hall bar samples were fabricated from high-quality tensilely strained HgTe layers by means of the improved lithographic processes. All samples were equipped with a top gate electrode, and some also with a modulation doping layer or a back gate electrode to modify the carrier density of the surface states on both sides of the HgTe layer. Due to the high sample quality, Landau levels could be well-resolved in standard transport measurements down to magnetic fields of less than 0.5T. High-resolution measurements of the Landau level dispersion with gate voltage and magnetic field allowed disentangling different transport channels. The main result here is that the upper (electron) branches of the two topological surface states contribute to transport in all experimentally relevant density regimes, while the hole branch is not accessible. Far in n-regime bulk conduction band states give a minor contribution to transport. More importantly, trivial bulk valence band holes come into play close to the charge neutrality point. Further in p-regime, the strong applied gate voltage leads to the formation of two-dimensional, massive hole states at the HgTe surface. The interplay of different states gives rise to rich physics: Top gate-back gate maps revealed that an anticrossing of Landau levels from the two topological surface states occurs at equal filling. A possible explanation for this effect is a weak hybridization of the surface states; however, future studies need to further clarify this point. Furthermore, the superposition of n-type topological and p-type trivial surface states leads to an intriguing Landau level dispersion. The good quantization of the Hall conductance in this situation indicates that the counterpropagating edge states interact with each other. The nature of this interaction will be the topic of further research. Part III of the thesis is focused on HgTe microstructures. These "channel samples" have a typical width of 0.5 to 4µm and a typical length of 5 to 80µm. The quality of these devices benefits particularly from the improved lithographic processes. As a result, the impurity mean free path of the topological surface state electrons is on the order of the device width and transport becomes semiballistic. This was verified by measuring the channel resistance in small magnetic fields in n-regime. The deflection of carriers towards the dissipative channel walls results in a pronounced peak in the magnetoresistance, which scales in a predictable manner with the channel width. To investigate transport effects due to mutual scattering of charge carriers, the differential resistance of channel samples was measured as a function of carrier temperature. Selective heating of the charge carriers - but not the lattice - was achieved by passing a heating current through the channel. Increasing the carrier temperature has two pronounced effects when the Fermi level is situated in proximity to the bulk valence band maximum where the density of states is large. First, when both topological surface state electrons and bulk holes are present, electron-hole scattering leads to a pronounced increase in resistance with increasing carrier temperature. Second, a thermally induced increase of the electron and hole carrier densities reduces the resistance again at higher temperatures. A model considering these two effects was developed, which can well reproduce the experimental results. Current heating experiments in zero-gap HgTe quantum wells and compressively strained HgTe layers are consistent with this model. These observations raise the question as to how electron-hole scattering may affect other transport properties of HgTe-based three-dimensional topological insulators, which is briefly discussed in the outlook.}, subject = {Topologischer Isolator}, language = {en} } @phdthesis{Harder2022, author = {Harder, Tristan H.}, title = {Topological Modes and Flatbands in Microcavity Exciton-Polariton Lattices}, doi = {10.25972/OPUS-25900}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-259008}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {The fascination of microcavity exciton-polaritons (polaritons) rests upon the combination of advanced technological control over both the III-V semiconductor material platform as well as the precise spectroscopic access to polaritonic states, which provide access to the investigation of open questions and complex phenomena due to the inherent nonlinearity and direct spectroscopic observables such as energy-resolved real and Fourier space information, pseudospin and coherence. The focus of this work was to advance the research area of polariton lattice simulators with a particular emphasis on their lasing properties. Following the brief introduction into the fundamental physics of polariton lattices in chapter 2, important aspects of the sample fabrication as well as the Fourier spectroscopy techniques used to investigate various features of these lattices were summarized in chapter 3. Here, the implementation of a spatial light modulator for advanced excitation schemes was presented. At the foundation of this work is the capability to confine polaritons into micropillars or microtraps resulting in discrete energy levels. By arranging these pillars or traps into various lattice geometries and ensuring coupling between neighbouring sites, polaritonic band structures were engineered. In chapter 4, the formation of a band structure was visualised in detail by investigating ribbons of honeycomb lattices. Here, the transition of the discrete energy levels of a single chain of microtraps to the fully developed band structure of a honeycomb lattice was observed. This study allows to design the size of individual domains in more complicated lattice geometries such that a description using band structures becomes feasible, as it revealed that a width of just six unit cells is sufficient to reproduce all characteristic features of the S band of a honeycomb lattice. In particular in the context of potential technological applications in the realms of lasing, the laser-like, coherent emission from polariton microcavities that can be achieved through the excitation of polariton condensates is intriguing. The condensation process is significantly altered in a lattice potential environment when compared to a planar microcavity. Therefore, an investigation of the polariton condensation process in a lattice with respect to the characteristics of the excitation laser, the exciton-photon detuning as well as the reduced trap distance that represents a key design parameter for polaritonic lattices was performed. Based on the demonstration of polariton condensation into multiple bands, the preferred condensation into a desired band was achieved by selecting the appropriate detuning. Additionally, a decreased condensation threshold in confined systems compared to a planar microcavity was revealed. In chapter 5, the influence of the peculiar feature of flatbands arising in certain lattice geometries, such as the Lieb and Kagome lattices, on polaritons and polariton condensates was investigated. Deviations from a lattice simulator described by a tight binding model that is solely based on nearest neighbour coupling cause a remaining dispersiveness of the flatbands along certain directions of the Brillouin zone. Therefore, the influence of the reduced trap distance on the dispersiveness of the flatbands was investigated and precise technological control over the flatbands was demonstrated. As next-nearest neighbour coupling is reduced drastically by increasing the distance between the corresponding traps, increasing the reduced trap distance enables to tune the S flatbands of both Lieb and Kagome lattices from dispersive bands to flatbands with a bandwidth on the order of the polariton linewidth. Additionally to technological control over the band structures, the controlled excitation of large condensates, single compact localized state (CLS) condensates as well as the resonant excitation of polaritons in a Lieb flatband were demonstrated. Furthermore, selective condensation into flatbands was realised. This combination of technological and spectroscopic control illustrates the capabilities of polariton lattice simulators and was used to study the coherence of flatband polariton condensates. Here, the ability to tune the dispersiveness from a dispersive band to an almost perfect flatband in combination with the selectivity of the excitation is particularly valuable. By exciting large flatband condensates, the increasing degree of localisation to a CLS with decreasing dispersiveness was demonstrated by measurements of first order spatial coherence. Furthermore, the first order temporal coherence of CLS condensates was increased from τ = 68 ps for a dispersive flatband, a value typically achieved in high-quality microcavity samples, to a remarkable τ = 459 ps in a flatband with a dispersiveness below the polarion linewidth. Corresponding to this drastic increase of the first order coherence time, a decrease of the second order temporal coherence function from g(2)(τ =0) = 1.062 to g(2)(0) = 1.035 was observed. Next to laser-like, coherent emission, polariton condensates can form vortex lattices. In this work, two distinct vortex lattices that can form in polariton condensates in Kagome flatbands were revealed. Furthermore, chiral, superfluid edge transport was realised by breaking the spatial symmetry through a localised excitation spot. This chirality was related to a change in the vortex orientation at the edge of the lattice and thus opens the path towards further investigations of symmetry breaking and chiral superfluid transport in Kagome lattices. Arguably the most influential concept in solid-state physics of the recent decades is the idea of topological order that has also provided a new degree of freedom to control the propagation of light. Therefore, in chapter 6, the interplay of topologically non-trivial band structures with polaritons, polariton condensates and lasing was emphasised. Firstly, a two-dimensional exciton-polariton topological insulator based on a honeycomb lattice was realised. Here, a topologically non-trivial band gap was opened at the Dirac points through a combination of TE-TM splitting of the photonic mode and Zeeman splitting of the excitonic mode. While the band gap is too small compared to the linewidth to be observed in the linear regime, the excitation of polariton condensates allowed to observe the characteristic, topologically protected, chiral edge modes that are robust against scattering at defects as well as lattice corners. This result represents a valuable step towards the investigation of non-linear and non-Hermitian topological physics, based on the inherent gain and loss of microcavities as well as the ability of polaritons to interact with each other. Apart from fundamental interest, the field of topological photonics is driven by the search of potential technological applications, where one direction is to advance the development of lasers. In this work, the starting point towards studying topological lasing was the Su-Schrieffer-Heeger (SSH) model, since it combines a simple and well-understood geometry with a large topological gap. The coherence properties of the topological edge defect of an SSH chain was studied in detail, revealing a promising degree of second order temporal coherence of g(2)(0) = 1.07 for a microlaser with a diameter of only d = 3.5 µm. In the context of topological lasing, the idea of using a propagating, topologically protected mode to ensure coherent coupling of laser arrays is particularly promising. Here, a topologically non-trivial interface mode between the two distinct domains of the crystalline topological insulator (CTI) was realised. After establishing selective lasing from this mode, the coherence properties were studied and coherence of a full, hexagonal interface comprised of 30 vertical-cavity surface-emitting lasers (VCSELs) was demonstrated. This result thus represents the first demonstration of a topological insulator VCSEL array, combining the compact size and convenient light collection of vertically emitting lasers with an in-plane topological protection. Finally, in chapter 7, an approach towards engineering the band structures of Lieb and honeycomb lattices by unbalancing the eigenenergies of the sites within each unit cell was presented. For Lieb lattices, this technique opens up a path towards controlling the coupling of a flatband to dispersive bands and could enable a detailed study of the influence of this coupling on the polariton flatband states. In an unbalanced honeycomb lattice, a quantum valley Hall boundary mode between two distinct, unbalanced honeycomb domains with permuted sites in the unit cells was demonstrated. This boundary mode could serve as the foundation for the realisation of a polariton quantum valley Hall effect with a truly topologically protected spin based on vortex charges. Modifying polariton lattices by unbalancing the eigenenergies of the sites that comprise a unit cell was thus identified as an additional, promising path for the future development of polariton lattice simulators.}, subject = {Exziton-Polariton}, language = {en} } @phdthesis{Eirich2022, author = {Eirich, Philipp}, title = {Accelerated non-Cartesian cardiovascular MR Imaging at 3T and 7T}, doi = {10.25972/OPUS-25397}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-253974}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In this work, accelerated non-Cartesian Magnetic Resonance Imaging (MRI) methods were established and applied to cardiovascular imaging (CMR) at different magnetic field strengths (3T and 7T). To enable rapid data acquisition, highly efficient spiral k-space trajectories were created. In addition, hybrid sampling patterns such as the twisting radial lines (TWIRL) k-space trajectory were studied. Imperfections of the dynamic gradient system of a MR scanner result in k-space sampling errors. Ultimately, these errors can lead to image artifacts in non-Cartesian acquisitions. Among other reasons such as an increased reconstruction complexity, they cause the lack of spiral sequences in clinical routine compared to standard Cartesian imaging. Therefore, the Gradient System Transfer Functions (GSTFs) of both scanners were determined and used for k-space trajectory correction in post-correction as well as in terms of a pre-emphasis. The GSTF pre-emphasis was implemented as a fully automatic procedure, which enabled a precise correction of arbitrary gradient waveforms for double-oblique slice orientations. Consequently, artifacts due to trajectory errors could be mitigated, which resulted in high image quality in non-Cartesian MRI. Additionally, the GSTF correction was validated by measuring pre-emphasized spiral gradient outputs, which showed high agreement with the theoretical gradient waveforms. Furthermore, it could be demonstrated that the performance of the GSTF correction is superior to a simple delay compensation approach. The developed pulse sequences were applied to gated as well as real-time CMR. Special focus lied on the implementation of a spiral imaging protocol to resolve the beating heart of animals and humans in real time and free breathing. In order to achieve real-time CMR with high spatiotemporal resolution, k-space undersampling was performed. For this reason, efficient sampling strategies were developed with the aim to facilitate compressed sensing (CS) during image reconstruction. The applied CS approach successfully removed aliasing artifacts and yielded high-resolution cardiac image series. Image reconstruction was performed offline in all cases such that the images were not available immediately after acquisition at the scanner. Spiral real-time CMR could be performed in free breathing, which led to an acquisition time of less than 1 minute for a whole short-axis stack. At 3T, the results were compared to the gold standard of electrocardiogram-gated Cartesian CMR in breath hold, which revealed similar values for important cardiovascular functional and volumetric parameters. This paves the way to an application of the developed framework in clinical routine of CMR. In addition, the spiral real-time protocol was transferred to swallowing and speech imaging at 3T, and first images were presented. The results were of high quality and confirm the straightforward utilization of the spiral sequence in other fields of MRI. In general, the GSTF correction yielded high-quality images at both field strengths, 3T and 7T. Off-resonance related blurring was mitigated by applying non-Cartesian readout gradients of short duration. At 7T, however, B1-inhomogeneity led to image artifacts in some cases. All in all, this work demonstrated great advances in accelerating the MRI process by combining efficient, undersampled non-Cartesian k-space coverage with CS reconstruction. Trajectory correction using the GSTF can be implemented at any scanner model and enables non-Cartesian imaging with high image quality. Especially MRI of dynamic processes greatly benefits from the presented rapid imaging approaches.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Tcakaev2023, author = {Tcakaev, Abdul-Vakhab}, title = {Soft X-ray Spectroscopic Study of Electronic and Magnetic Properties of Magnetic Topological Insulators}, doi = {10.25972/OPUS-30378}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-303786}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {After the discovery of three-dimensional topological insulators (TIs), such as tetradymite chalcogenides Bi\$_2\$Se\$_3\$, Bi\$_2\$Te\$_3\$ and Sb\$_2\$Te\$_3\$ - a new class of quantum materials characterized by their unique surface electronic properties - the solid state community got focused on topological states that are driven by strong electronic correlations and magnetism. An important material class is the magnetic TI (MTI) exhibiting the quantum anomalous Hall (QAH) effect, i.e. a dissipationless quantized edge-state transport in the absence of external magnetic field, originating from the interplay between ferromagnetism and a topologically non-trivial band structure. The unprecedented opportunities offered by these new exotic materials open a new avenue for the development of low-dissipation electronics, spintronics, and quantum computation. However, the major concern with QAH effect is its extremely low onset temperature, limiting its practical application. To resolve this problem, a comprehensive understanding of the microscopic origin of the underlying ferromagnetism is necessary. V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ are the two prototypical systems that have been widely studied as realizations of the QAH state. Finding microscopic differences between the strongly correlated V and Cr impurities would help finding a relevant model of ferromagnetic coupling and eventually provide better control of the QAH effect in these systems. Therefore, this thesis first focuses on the V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ systems, to better understand these differences. Exploiting the unique capabilities of x-ray absorption spectroscopy and magnetic circular dichroism (XAS/XMCD), combined with advanced modeling based on multiplet ligand-field theory (MLFT), we provide a detailed microscopic insight into the local electronic and magnetic properties of these systems and determine microscopic parameters crucial for the comparison with theoretical models, which include the \$d\$-shell filling, spin and orbital magnetic moments. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from a purely ionic 3+ configuration. Our findings indicate the importance of the Te \$5p\$ states for the ferromagnetism in (Bi, Sb)\$_2\$Te\$_3\$ and favor magnetic coupling mechanisms involving \$pd\$-exchange. Using state-of-the-art density functional theory (DFT) calculations in combination with XMCD and resonant photoelectron spectroscopy (resPES), we reveal the important role of the \$3d\$ impurity states in mediating magnetic exchange coupling. Our calculations illustrate that the kind and strength of the exchange coupling varies with the impurity \$3d\$-shell occupation. We find a weakening of ferromagnetic properties upon the increase of doping concentration, as well as with the substitution of Bi at the Sb site. Finally, we qualitatively describe the origin of the induced magnetic moments at the Te and Sb sites in the host lattice and discuss their role in mediating a robust ferromagnetism based on a \$pd\$-exchange interaction scenario. Our findings reveal important clues to designing higher \$T_{\text{C}}\$ MTIs. Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of TIs, which is favorable for the realization of the high-temperature QAH effect. Therefore, we have further focused on Eu-doped Bi\$_2\$Te\$_3\$ and scrutinized whether the conditions for formation of a substantial gap in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the \$M_{4,5}\$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu\$^{2+}\$ valence, unlike most other rare earth elements, and confirms a large magnetic moment. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic ordering. This is in good agreement with DFT results, which predict AFM interactions between the Eu impurities due to the direct overlap of the impurity wave functions. Our results support the notion of antiferromagnetism coexisting with topological surface states in rare-earth doped Bi\$_2\$Te\$_3\$ and corroborate the potential of such doping to result in an antiferromagnetic TI with exotic quantum properties. The doping with impurities introduces disorder detrimental for the QAH effect, which may be avoided in stoichiometric, well-ordered magnetic compounds. In the last part of the thesis we have investigated the recently discovered intrinsic magnetic TI (IMTI) MnBi\$_6\$Te\$_{10}\$, where we have uncovered robust ferromagnetism with \$T_{\text{C}} \approx 12\$ K and connected its origin to the Mn/Bi intermixing. Our measurements reveal a magnetically intact surface with a large moment, and with FM properties similar to the bulk, which makes MnBi\$_6\$Te\$_{10}\$ a promising candidate for the QAH effect at elevated temperatures. Moreover, using an advanced ab initio MLFT approach we have determined the ground-state properties of Mn and revealed a predominant contribution of the \$d^5\$ configuration to the ground state, resulting in a \$d\$-shell electron occupation \$n_d = 5.31\$ and a large magnetic moment, in excellent agreement with our DFT calculations and the bulk magnetometry data. Our results together with first principle calculations based on the DFT-GGA\$+U\$, performed by our collaborators, suggest that carefully engineered intermixing plays a crucial role in achieving a robust long-range FM order and therefore could be the key for achieving enhanced QAH effect properties. We expect our findings to aid better understanding of MTIs, which is essential to help increasing the temperature of the QAH effect, thus facilitating the realization of low-power electronics in the future.}, subject = {Topologischer Isolator}, language = {en} } @article{MuellerGraetzBallesetal.2021, author = {M{\"u}ller, Dominik and Graetz, Jonas and Balles, Andreas and Stier, Simon and Hanke, Randolf and Fella, Christian}, title = {Laboratory-Based Nano-Computed Tomography and Examples of Its Application in the Field of Materials Research}, series = {Crystals}, volume = {11}, journal = {Crystals}, number = {6}, issn = {2073-4352}, doi = {10.3390/cryst11060677}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-241048}, year = {2021}, abstract = {In a comprehensive study, we demonstrate the performance and typical application scenarios for laboratory-based nano-computed tomography in materials research on various samples. Specifically, we focus on a projection magnification system with a nano focus source. The imaging resolution is quantified with common 2D test structures and validated in 3D applications by means of the Fourier Shell Correlation. As representative application examples from nowadays material research, we show metallization processes in multilayer integrated circuits, aging in lithium battery electrodes, and volumetric of metallic sub-micrometer fillers of composites. Thus, the laboratory system provides the unique possibility to image non-destructively structures in the range of 170-190 nanometers, even for high-density materials.}, language = {en} } @phdthesis{Gruene2022, author = {Gr{\"u}ne, Jeannine}, title = {Spin States and Efficiency-Limiting Pathways in Optoelectronic Materials and Devices}, doi = {10.25972/OPUS-29340}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-293405}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {This thesis addresses the identification and characterization of spin states in optoelectronic materials and devices using multiple spin-sensitive techniques. For this purpose, a systematic study focussing on triplet states as well as associated loss pathways and excited state kinetics was carried out. The research was based on comparing a range of donor:acceptor systems, reaching from organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) to organic photovoltaics (OPV) employing fullerene and multiple non-fullerene acceptors (NFAs). By developing new strategies, e.g., appropriate modeling, new magnetic resonance techniques and experimental frameworks, the influence of spin states in the fundamental processes of organic semiconductors has been investigated. Thereby, the combination of techniques based on the principle of electron paramagnetic resonance (EPR), in particular transient EPR (trEPR) and optically detected magnetic resonance (ODMR), with all-optical methods, such as transient electroluminescence (trEL) and transient absorption (TA), has been employed. As a result, excited spin states, especially molecular and charge transfer (CT) states, were investigated in terms of kinetic behavior and associated pathways, which revealed a significant impact of triplet states on efficiency-limiting processes in both optoelectronic applications.}, subject = {Elektronenspinresonanz}, language = {en} } @article{UenzelmannBentmannFiggemeieretal.2021, author = {{\"U}nzelmann, M. and Bentmann, H. and Figgemeier, T. and Eck, P. and Neu, J. N. and Geldiyev, B. and Diekmann, F. and Rohlf, S. and Buck, J. and Hoesch, M. and Kall{\"a}ne, M. and Rossnagel, K. and Thomale, R. and Siegrist, T. and Sangiovanni, G. and Di Sante, D. and Reinert, F.}, title = {Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs}, series = {Nature Communications}, volume = {12}, journal = {Nature Communications}, number = {1}, doi = {10.1038/s41467-021-23727-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-260719}, year = {2021}, abstract = {Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids. Weyl semimetals exhibit Berry flux monopoles in momentum-space, but direct experimental evidence has remained elusive. Here, the authors reveal topologically non-trivial winding of the orbital-angular-momentum at the Weyl nodes and a chirality-dependent spin-angular-momentum of the Weyl bands, as a direct signature of the Berry flux monopoles in TaAs.}, language = {en} } @phdthesis{Lang2017, author = {Lang, Jean-Nicolas Olivier}, title = {Automation of electroweak NLO corrections in general models}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-154426}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {The thesis deals with the automated generation and efficient evaluation of scattering amplitudes in general relativistic quantum field theories at one-loop order in perturbation theory. At the present time we lack signals beyond the Standard Model which, in the past, have guided the high-energy physics community, and ultimately led to the discovery of new physics phenomena. In the future, precision tests could acquire this guiding role by systematically probing the Standard Model and constraining Beyond the Standard Model theories. As current experimental constraints strongly favour Standard Model-like theories, only small deviations with respect to the Standard Model are expected which need to be studied in detail. The required precision demands one-loop corrections in all future analyses, ideally in a fully automated way, allowing to test a variety of observables in different models and in an effective field theory approach. In the process of achieving this goal we have developed an enhanced version of the tool Recola and on this basis the generalization Recola2. These tools represent fully automated tree- and one-loop-amplitude providers for the Standard Model, or in the case of Recola2 for general models. Concerning the algorithm, we use a purely numerical and fully recursive approach allowing for extreme calculations of yet unmatched complexity. Recola has led to the first computation involving 9-point functions. Beyond the Standard Model theories and Effective Field theories are integrated into the Recola2 framework as model files. Renormalized model files are produced with the newly developed tool Rept1l, which can perform the renormalization in a fully automated way, starting from nothing but Feynman rules. In view of validation, we have extended Recola2 to new gauges such as the Background-Field Method and the class of Rxi gauges. In particular, the Background-Field Method formulation for new theories serves as an automated validation, and is very useful in practical calculations and the formulation of renormalization conditions. We have applied the system to produce the first results for Higgs-boson production in Higgs strahlung and vector-boson fusion in the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model. All in all, we have laid the foundation for an automated generation and computation of one-loop amplitudes within a large class of phenomenologically interesting theories. Furthermore, we enable the use of our system via a very flexible and dynamic control which does not require any intermediate intervention.}, subject = {Standardmodell }, language = {en} } @article{KuemmelLindenberger2020, author = {K{\"u}mmel, Reiner and Lindenberger, Dietmar}, title = {Energy, entropy, constraints, and creativity in economic growth and crises}, series = {Entropy}, volume = {22}, journal = {Entropy}, number = {10}, issn = {1099-4300}, doi = {10.3390/e22101156}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-216275}, year = {2020}, abstract = {The neoclassical mainstream theory of economic growth does not care about the First and the Second Law of Thermodynamics. It usually considers only capital and labor as the factors that produce the wealth of modern industrial economies. If energy is taken into account as a factor of production, its economic weight, that is its output elasticity, is assigned a meager magnitude of roughly 5 percent, according to the neoclassical cost-share theorem. Because of that, neoclassical economics has the problems of the "Solow Residual", which is the big difference between observed and computed economic growth, and of the failure to explain the economic recessions since World War 2 by the variations of the production factors. Having recalled these problems, we point out that technological constraints on factor combinations have been overlooked in the derivation of the cost-share theorem. Biophysical analyses of economic growth that disregard this theorem and mend the neoclassical deficiencies are sketched. They show that energy's output elasticity is much larger than its cost share and elucidate the existence of bidirectional causality between energy conversion and economic growth. This helps to understand how economic crises have been triggered and overcome by supply-side and demand-side actions. Human creativity changes the state of economic systems. We discuss the challenges to it by the risks from politics and markets in conjunction with energy sources and technologies, and by the constraints that the emissions of particles and heat from entropy production impose on industrial growth in the biosphere.}, language = {en} } @phdthesis{Kissner2022, author = {Kißner, Katharina}, title = {Manipulation of electronic properties in strongly correlated Cerium-based surface alloys}, doi = {10.25972/OPUS-27306}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-273067}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Photoelectron spectroscopy proves as a versatile tool for investigating various aspects of the electronic structure in strongly correlated electron systems. Influencing the manifestation of strong correlation in Ce-based surface alloys is the main task of this work. It is shown, that the manifestation of the Kondo ground state is influenced by a multitude of parameters such as the choice of the metal binding partner in binary Ce compounds, the surface alloy layer thickness and accompanying variations in the lattice structure as well as the interfaces to substrate or vacuum. Gaining access to these parameters allows to directly influence essential state variables, such as the f level occupancy nf or the Kondo temperature TK. The center of this work are the intermetallic thin films of CePt5/Pt(111) and CeAgx/Ag(111). By utilizing different excitation energies, photoemission spectroscopy provides access to characteristic features of Kondo physics in the valence band, such as the Kondo resonance and its spin-orbit partner at the Fermi level, as well as the multiplet structure of the Ce 3d core levels. In this work both approaches are applied to CePt5/Pt(111) to determine nf and TK for a variety of surface alloy layer thicknesses. A temperature dependent study of the Ce 3d core levels allows to determine the systems TK for the different layer thicknesses. This leads to TK ≈200-270K in the thin layer thickness regime and TK >280K for larger layer thicknesses. These results are confirmed by fitting the Ce 3d multiplet based on the Gunnarsson-Sch{\"o}nhammer formalism for core level spectroscopy and additionally by valence band photoemission spectra of the respective Kondo resonances. The influence of varying layer thickness on the manifestation of strong correlation is subsequently studied for the surface alloy CeAgx/Ag(111). Furthermore, the heavy element Bi is added, to investigate the effects of strong spin-orbit coupling on the electronic structure of the surface alloy.}, subject = {Korrelation}, language = {en} } @article{WeissenseelGottschollBoennighausenetal.2021, author = {Weissenseel, Sebastian and Gottscholl, Andreas and B{\"o}nnighausen, Rebecca and Dyakonov, Vladimir and Sperlich, Andreas}, title = {Long-lived spin-polarized intermolecular exciplex states in thermally activated delayed fluorescence-based organic light-emitting diodes}, series = {Science Advances}, volume = {7}, journal = {Science Advances}, number = {47}, doi = {10.1126/sciadv.abj9961}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-265508}, year = {2021}, abstract = {Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are pivotal because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). To explore the underlying process, we apply a spin-resonance spectral hole-burning technique to probe electroluminescence. We find that the triplet exciplex states in OLEDs are highly spin-polarized and show that these states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even be coherently manipulated on a spin-spin relaxation time scale T-2* of 30 ns. Crucially, we obtain the characteristic triplet exciplex spin-lattice relaxation time T-1 in the range of 50 mu s, which far exceeds the RISC time. We conclude that slow spin relaxation rather than RISC is an efficiency-limiting step for intermolecular donor:acceptor systems. Finding TADF emitters with faster spin relaxation will benefit this type of TADF OLEDs.}, language = {en} } @article{WinterAndelovicKampfetal.2021, author = {Winter, Patrick M. and Andelovic, Kristina and Kampf, Thomas and Hansmann, Jan and Jakob, Peter Michael and Bauer, Wolfgang Rudolf and Zernecke, Alma and Herold, Volker}, title = {Simultaneous measurements of 3D wall shear stress and pulse wave velocity in the murine aortic arch}, series = {Journal of Cardiovascular Magnetic Resonance}, volume = {23}, journal = {Journal of Cardiovascular Magnetic Resonance}, number = {1}, doi = {10.1186/s12968-021-00725-4}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-259152}, pages = {34}, year = {2021}, abstract = {Purpose Wall shear stress (WSS) and pulse wave velocity (PWV) are important parameters to characterize blood flow in the vessel wall. Their quantification with flow-sensitive phase-contrast (PC) cardiovascular magnetic resonance (CMR), however, is time-consuming. Furthermore, the measurement of WSS requires high spatial resolution, whereas high temporal resolution is necessary for PWV measurements. For these reasons, PWV and WSS are challenging to measure in one CMR session, making it difficult to directly compare these parameters. By using a retrospective approach with a flexible reconstruction framework, we here aimed to simultaneously assess both PWV and WSS in the murine aortic arch from the same 4D flow measurement. Methods Flow was measured in the aortic arch of 18-week-old wildtype (n = 5) and ApoE\(^{-/-}\) mice (n = 5) with a self-navigated radial 4D-PC-CMR sequence. Retrospective data analysis was used to reconstruct the same dataset either at low spatial and high temporal resolution (PWV analysis) or high spatial and low temporal resolution (WSS analysis). To assess WSS, the aortic lumen was labeled by semi-automatically segmenting the reconstruction with high spatial resolution. WSS was determined from the spatial velocity gradients at the lumen surface. For calculation of the PWV, segmentation data was interpolated along the temporal dimension. Subsequently, PWV was quantified from the through-plane flow data using the multiple-points transit-time method. Reconstructions with varying frame rates and spatial resolutions were performed to investigate the influence of spatiotemporal resolution on the PWV and WSS quantification. Results 4D flow measurements were conducted in an acquisition time of only 35 min. Increased peak flow and peak WSS values and lower errors in PWV estimation were observed in the reconstructions with high temporal resolution. Aortic PWV was significantly increased in ApoE\(^{-/-}\) mice compared to the control group (1.7 ± 0.2 versus 2.6 ± 0.2 m/s, p < 0.001). Mean WSS magnitude values averaged over the aortic arch were (1.17 ± 0.07) N/m\(^2\) in wildtype mice and (1.27 ± 0.10) N/m\(^2\) in ApoE\(^{-/-}\) mice. Conclusion The post processing algorithm using the flexible reconstruction framework developed in this study permitted quantification of global PWV and 3D-WSS in a single acquisition. The possibility to assess both parameters in only 35 min will markedly improve the analyses and information content of in vivo measurements.}, language = {en} } @phdthesis{Balzer2018, author = {Balzer, Christian}, title = {Adsorption-Induced Deformation of Nanoporous Materials — in-situ Dilatometry and Modeling}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-157145}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {The goal of this work is to improve the understanding of adsorption-induced deformation in nanoporous (and in particular microporous) materials in order to explore its potential for material characterization and provide guidelines for related technical applications such as adsorption-driven actuation. For this purpose this work combines in-situ dilatometry measurements with in-depth modeling of the obtained adsorption-induced strains. A major advantage with respect to previous studies is the combination of the dilatometric setup and a commercial sorption instrument resulting in high quality adsorption and strain isotherms. The considered model materials are (activated and thermally annealed) carbon xerogels, a sintered silica aerogel, a sintered hierarchical structured porous silica and binderless zeolites of type LTA and FAU; this selection covers micro-, meso- and macroporous as well as ordered and disordered model materials. All sample materials were characterized by scanning electron microscopy, gas adsorption and sound velocity measurements. In-situ dilatometry measurements on mesoporous model materials were performed for the adsorption of N2 at 77 K, while microporous model materials were also investigated for CO2 adsorption at 273 K, Ar adsorption at 77 K and H2O adsorption at 298 K. Within this work the available in-situ dilatometry setup was revised to improve resolution and reproducibility of measurements of small strains at low relative pressures, which are of particular relevance for microporous materials. The obtained experimental adsorption and strain isotherms of the hierarchical structured porous silica and a micro-macroporous carbon xerogel were quantitatively analyzed based on the adsorption stress model; this approach, originally proposed by Ravikovitch and Neimark, was extended for anisotropic pore geometries within this work. While the adsorption in silica mesopores could be well described by the classical and analytical theory of Derjaguin, Broekhoff and de Boer, the adsorption in carbon micropores required for comprehensive nonlocal density functional theory calculations. To connect adsorption-induced stresses and strains, furthermore mechanical models for the respective model materials were derived. The resulting theoretical framework of adsorption, adsorption stress and mechanical model was applied to the experimental data yielding structural and mechanical information about the model materials investigated, i.e., pore size or pore size distribution, respectively, and mechanical moduli of the porous matrix and the nonporous solid skeleton. The derived structural and mechanical properties of the model materials were found to be consistent with independent measurements and/or literature values. Noteworthy, the proposed extension of the adsorption stress model proved to be crucial for the correct description of the experimental data. Furthermore, it could be shown that the adsorption-induced deformation of disordered mesoporous aero-/xerogel structures follows qualitatively the same mechanisms obtained for the ordered hierarchical structured porous silica. However, respective quantitative modeling proved to be challenging due to the ill-shaped pore geometry of aero-/xerogels; good agreement between model and experiment could only be achieved for the filled pore regime of the adsorption isotherm and the relative pressure range of monolayer formation. In the intermediate regime of multilayer formation a more complex model than the one proposed here is required to correctly describe stress related to the curved adsorbate-adsorptive interface. Notably, for micro-mesoporous carbon xerogels it could be shown that micro- and mesopore related strain mechanisms superimpose one another. The strain isotherms of the zeolites were only qualitatively evaluated. The result for the FAU type zeolite is in good agreement with other experiments reported in literature and the theoretical understanding derived from the adsorption stress model. On the contrary, the strain isotherm of the LTA type zeolite is rather exceptional as it shows monotonic expansion over the whole relative pressure range. Qualitatively this type of strain isotherm can also be explained by the adsorption stress model, but a respective quantitative analysis is beyond the scope of this work. In summary, the analysis of the model materials' adsorption-induced strains proved to be a suitable tool to obtain information on their structural and mechanical properties including the stiffness of the nonporous solid skeleton. Investigations on the carbon xerogels modified by activation and thermal annealing revealed that adsorption-induced deformation is particularly suited to analyze even small changes of carbon micropore structures.}, subject = {Nanopor{\"o}ser Stoff}, language = {en} } @phdthesis{Kampf2018, author = {Kampf, Thomas}, title = {Quantifizierung myokardialer Mikrostruktur und Perfusion mittels longitudinaler NMR Relaxation}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-174261}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Ziel der Arbeit war es die Quantifizierung funktioneller bzw. mikrostruktureller Parameter des Herzmuskels mit Hilfe T1-basierter Methoden zu verbessern. Diese Methoden basieren darauf, die gew{\"u}nschte Information durch eine geeignete Pr{\"a}paration der Magnetisierung bzw. durch die Gabe von Kontrastmittel in den Zeitverlauf der longitudinalen Relaxation zu kodieren. Aus der {\"A}nderung der Relaxationszeit l{\"a}ßt sich dann die gew{\"u}nschte Information bestimmen. Daf{\"u}r sollte sowohl der Einfluß der Anatomie als auch derjenige der Meßmethodik auf die Bestimmung der longitudinalen Relaxationszeit und damit auf die Quantifizierung der Funktion bzw. Mikrostrukturparameter untersucht werden. Speziell der Einfluß der Bildgebungssequenz f{\"u}hrt dazu, daß nur eine scheinbare Relaxationszeit gemessen wird. W{\"a}hrend dies keinen Einfluß auf die T1-basierte Bestimmung der untersuchten Mikrostrukturparameter hatte, ergab sich f{\"u}r die Perfusionsquantifizierung eine deutliche Abh{\"a}ngigkeit von den Parametern der verwendeten IRLL-Sequenz. Um diesen Einfluß gerecht zu werden, wurden an die Meßmethodik angepaßte Gleichungen zur Bestimmung der Perfusion gefunden mit denen die systematischen Abweichungen korrigiert werden k{\"o}nnen. Zus{\"a}tzlich reduzieren die angepaßten Gleichungen die Anforderungen bez{\"u}glich der Inversionsqualit{\"a}t im schichtselektiven Experiment. Dies wurde in einem weiteren Projekt bei der Bestimmung der Nierenperfusion im Mausmodell ausgenutzt. Neben der Untersuchung der Auswirkungen der Meßmethode wurde auch der Einfluß der anatomischen Besonderheiten des Blutkreislaufs am Herzen auf die Parameterquantifizierung mittels T1-basierter Methoden untersucht. Es konnte gezeigt werden, daß auf Grund der Anatomie des Herzens bei typischen Orientierungen der Bildgebungsschicht, auch bei der schichtselektiven Inversionspr{\"a}paration der Magnetisierung des Herzmuskels ein Anteil des Blutpools invertiert wird. Daraus folgt, daß die vereinfachende Annahme, nach welcher bei schichtselektiver Pr{\"a}paration in Folge von Perfusion nur Blut mit Gleichgewichtsmagnetisierung den Herzmuskel erreicht, nicht erf{\"u}llt ist. Es konnte gezeigt werden, daß dies bei Perfusion zu einer deutlichen Untersch{\"a}tzung der berechneten Perfusionswertes f{\"u}hrt. Um mit diesem Problem umgehen zu k{\"o}nnen, wurde aufbauend auf einem vereinfachten Modell der zeitlichen Entwicklung der Blutmagnetisierung eine Korrektur f{\"u}r die Bestimmung der Perfusionswerte gefunden welche den Einfluß der anatomischen Besonderheiten ber{\"u}cksichtigt. Das f{\"u}r die Perfusionskorrektur eingef{\"u}hrte Model prognostiziert ebenso, daß auch bei schichtselektiver Inversion die T1-basierte Bestimmung der untersuchten Mikrostrukturparameter von der Perfusion abh{\"a}ngig wird und eine systematische {\"U}bersch{\"a}tzung der quantifizierten Werte verursacht. Da die Perfusion im Kleintier deutlich h{\"o}her ist als im Menschen, ist dieser Einfluß besonders in der pr{\"a}klinischen Forschung zu beachten. So k{\"o}nnen dort allein durch verminderte Perfusion deutliche {\"A}nderungen in den bestimmten Werten der Mikrostrukturparameter erzeugt werden, welche zu einer fehlerhaften Interpretation der Ergebnisse f{\"u}hren und somit ein falsches Bild f{\"u}r die Vorg{\"a}nge im Herzmuskel suggerieren. Dabei best{\"a}tigt der Vergleich mit experimentellen Ergebnissen aus der Literatur die Vorhersagen f{\"u}r das Rattenmodell. Beim Menschen ist der prognostizierte Effekt deutlich kleiner. Der prognostizierte Fehler bspw. im RBV-Wert liegt in diesem Fall bei etwa 10\% und wird {\"u}blicherweise in der aktuellen Forschung vernachl{\"a}ssigt. Inwieweit dies in er klinischen Forschung gerechtfertigt ist, muß in weiteren Untersuchungen gekl{\"a}rt werden. Den untersuchten Methoden zur Bestimmung von funktionellen und mikrostrukturellen Parametern ist gemein, daß sie eine exakte Quantifizierung der longitudinalen Relaxationszeit T1 ben{\"o}tigen. Dabei ist im Kleintierbereich die klassische IRLL-Methode als zuverl{\"a}ssige Sequenz zur T1-Quantifizierung etabliert. In der klinischen Bildgebung werden auf Grund der unterschiedlichen Zeitskalen und anderer technischer Voraussetzungen andere Anforderungen an die Datenakquisition gestellt. Dabei hat in den letzten Jahren die MOLLI-Sequenz große Verbreitung gefunden. Sie ist eine Abwandlung der IRLL-Sequenz, bei der mit einer bSSFP-Bildgebungssequenz getriggert ganze Bilder w{\"a}hrend eines Herzschlages aufgenommen werden. Die MOLLI-Sequenz reagiert dabei empfindlich auf die Wartezeiten zwischen den einzelnen Transienten. Um mit diese Problematik in den Griff zu bekommen und gleichzeitig die Meßzeit verk{\"u}rzen zu k{\"o}nnen wurde eine neue Methode zum Fitten der Daten entwickelt, welche die Abh{\"a}ngigkeit der scheinbaren Relaxationszeit von der Wartezeit zwischen den einzelnen Transienten, sowie der mittleren Herzrate fast vollst{\"a}ndig eliminiert. Diese Methode liefert f{\"u}r das ganze klinisch Spektrum an erwarteten T1-Zeiten, vor und nach Kontrastmittelgabe, stabile Ergebnisse und erlaubte ein deutliche Verk{\"u}rzung der Meßzeit, ohne die Anzahl der aufgenommenen Meßzeitpunkte zu reduzieren. Dies wurde in einer initialen klinischen Studie genutzt, um ECV-Werte in Patienten zu bestimmen. Ein Nachteil der Verwendung der MOLLI-Sequenz ist, daß nur die scheinbare Relaxationszeit aus den Fit der Meßdaten bestimmt wird. Die standardm{\"a}ßig genutzte Korrektur benutzt aber dem gefitteten Wert der Gleichgewichtsmagnetisierung um den wahren T1-Wert zu bestimmen. Somit ist es f{\"u}r die Bestimmung des T1-Wertes notwendig, die Qualit{\"a}t der Inversionspr{\"a}paration zu kennen. Auf Basis der neuen Fitmethode wurde eine Anpassung der MOLLI-Sequenz demonstriert, welche die Bestimmung der Gleichgewichtsmagnetisierung unabh{\"a}ngig von der Qualit{\"a}t der Inversionspr{\"a}paration erlaubt. Daf{\"u}r verl{\"a}ngert sich die Meßdauer lediglich um einen Herzschlag um in geeigneter Weise ein zus{\"a}tzliches Bild aufnehmen zu k{\"o}nnen. Abschließend wurde in dieser Arbeit der Signal-Zeit-Verlauf der MOLLI-Sequenz eingehend theoretische untersucht um ein besseres Verst{\"a}ndnis der getriggerten IRLL-Sequenzen zu entwickeln. In diesem Zusammenhang konnte eine einfache Interpretation der scheinbaren Relaxationszeit gefunden werden. Ebenso konnte erkl{\"a}rt werden, warum die f{\"u}r ungetriggerte IRLL-Sequenzen abgeleitete Korrekturgleichung auch im getriggerten Fall erstaunlich gute Ergebnisse liefert. Weiterhin konnten Fehlerquellen f{\"u}r die verbleibenden Abweichungen identifiziert werden, welche als Ausgangspunkt f{\"u}r die Ableitung verbesserter Korrekturgleichungen genutzt werden k{\"o}nnen.}, subject = {Kernspintomographie}, language = {de} } @phdthesis{Finkenberg2018, author = {Finkenberg, Frank}, title = {Flipped Classroom im Physikunterricht}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-164146}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {In der Unterrichtsmethode Flipped Classroom sind schulische und h{\"a}usliche Aktivit{\"a}ten vertauscht. Instruktionale Elemente werden in online verf{\"u}gbare Lernvideos ausgelagert, welche die Sch{\"u}ler als h{\"a}usliche Vorbereitung ansehen. Im Unterricht stehen dann sch{\"u}lerzentrierte T{\"a}tigkeiten im Vordergrund, in denen die Sch{\"u}ler ihr Wissen anwenden und vertiefen k{\"o}nnen. Durch die Auslagerung von Inputphasen wandelt sich die Rolle des Lehrers vom Instructor zum Lernbegleiter. Die vorliegende quasi-experimentelle Studie im Pre-/Postdesign mit Kontrollgruppe untersuchte die Wirkungen des Flipped Classroom in Physikkursen der Oberstufe (Grundkursniveau) an zwei deutschen Gymnasien mit N = 151 Sch{\"u}lerinnen und Sch{\"u}lern. Acht Physikkurse der 11. Jahrgangsstufe nahmen an der Studie teil, die sich {\"u}ber einen Zeitraum von zwei Schuljahren erstreckte (2015/16 und 2016/17). Vier der f{\"u}nf teilnehmenden Lehrkr{\"a}fte unterrichteten sowohl einen Kontroll- als auch einen Treatmentkurs. S{\"a}mtliche Lernvideos wurden von den Lehrkr{\"a}ften selbst erstellt. Dabei integrierten sie reale Experimente, um dem Anspruch physikauthentischen Unterrichts gerecht zu werden. Die Forschungsfragen richteten sich sowohl auf die Leistung in einem Fachwissenstest als auch auf affektive Lernmerkmale wie die Motivation, das Interesse und das Selbstkonzept. Zus{\"a}tzlich wurden die wahrgenommene Lehrerunterst{\"u}tzung und das Hausaufgabenverhalten untersucht. Die Anwendung von Flipped Classroom im Physikunterricht zeigte gr{\"o}ßtenteils positive Effekte. Die Sch{\"u}lerinnen und Sch{\"u}ler im Flipped Classroom hatten einen h{\"o}heren kognitiven Lernzuwachs und ein besseres Selbstkonzept als ihre Mitsch{\"u}ler, die traditionell unterrichtet wurden. Das Leistungsniveau und das Geschlecht der Sch{\"u}lerinnen und Sch{\"u}ler hatten dabei keinen Einfluss auf diese Effekte. W{\"a}hrend die Motivation, sich mit Physik zu besch{\"a}ftigen, in der Kontrollgruppe sank, blieb sie in der Treatmentgruppe auf konstantem Niveau. Bei genauerem Blick zeigte sich, dass die Motivation bei Sch{\"u}lerinnen im Flipped Classroom anstieg, bei Sch{\"u}lerinnen im traditionellen Unterricht jedoch abnahm. Das Interesse am Unterrichtsfach Physik wurde in beiden Gruppen geringer. Sowohl die wahrgenommene Lehrerunterst{\"u}tzung als auch die Hausaufgabendauer blieben in beiden Gruppen zwischen Pre- und Posttest unver{\"a}ndert. Die Hausaufgabendisziplin war im Flipped Classroom jedoch deutlich h{\"o}her, was zeigt, dass die Sch{\"u}lerinnen und Sch{\"u}ler eher bereit waren, sich instruktionale Lernvideos anzusehen als klassische Hausaufgaben zu bearbeiten.}, subject = {Physikunterricht}, language = {de} } @phdthesis{Matthaiakakis2021, author = {Matthaiakakis, Ioannis}, title = {Hydrodynamics in Solid State Systems and the AdS/CFT correspondence}, doi = {10.25972/OPUS-24439}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244390}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {We employ the AdS/CFT correspondence and hydrodynamics to analyze the transport properties of \(2+1\) dimensional electron fluids. In this way, we use theoretical methods from both condensed matter and high-energy physics to derive tangible predictions that are directly verifiable in experiment. The first research topic we consider is strongly-coupled electron fluids. Motivated by early results by Gurzhi on the transport properties of weakly coupled fluids, we consider whether similar properties are manifest in strongly coupled fluids. More specifically, we focus on the hydrodynamic tail of the Gurzhi effect: A decrease in fluid resistance with increasing temperature due to the formation of a Poiseuille flow of electrons in the sample. We show that the hydrodynamic tail of the Gurzhi effect is also realized in strongly coupled and fully relativistic fluids, but with modified quantitative features. Namely, strongly-coupled fluids always exhibit a smaller resistance than weakly coupled ones and are, thus, far more efficient conductors. We also suggest that the coupling dependence of the resistance can be used to measure the coupling strength of the fluid. In view of these measurements, we provide analytical results for the resistance as a function of the shear viscosity over entropy density \(\eta/s\) of the fluid. \(\eta/s\) is itself a known function of the coupling strength in the weak and infinite coupling limits. In further analysis for strongly-coupled fluids, we propose a novel strongly coupled Dirac material based on a kagome lattice, Scandium-substituted Herbertsmithite (ScHb). The large coupling strength of this material, as well as its Dirac nature, provides us with theoretical and experimental access to non-perturbative relativistic and quantum critical physics. A highly suitable method for analyzing such a material's transport properties is the AdS/CFT correspondence. Concretely, using AdS/CFT we derive an estimate for ScHb's \(\eta/s\) and show that it takes a value much smaller than that observed in weakly coupled materials. In turn, the smallness of \(\eta/s\) implies that ScHb's Reynolds number, \(Re\), is large. In fact, \(Re\) is large enough for turbulence, the most prevalent feature of fluids in nature, to make its appearance for the first time in electronic fluids. Switching gears, we proceed to the second research topic considered in this thesis: Weakly coupled parity-breaking electron fluids. More precisely, we analyze the quantitative and qualitative changes to the classical Hall effect, for electrons propagating hydrodynamically in a lead. Apart from the Lorentz force, a parity-breaking fluid's motion is also impacted by the Hall-viscous force; the shear-stress force induced by the Hall-viscosity. We show that the interplay of these two forces leads to a hydrodynamic Hall voltage with non-linear dependence on the magnetic field. More importantly, the Lorentz and Hall-viscous forces become equal at a non-vanishing magnetic field, leading to a trivial hydrodynamic Hall voltage. Moreover, for small magnetic fields we provide analytic results for the dependence of the hydrodynamic Hall voltage on all experimentally-tuned parameters of our simulations, such as temperature and density. These dependences, along with the zero of the hydrodynamic Hall voltage, are distinct features of hydrodynamic transport and can be used to verify our predictions in experiments. Last but not least, we consider how a distinctly electronic property, spin, can be included into the hydrodynamic framework. In particular, we construct an effective action for non-dissipative spin hydrodynamics up to first order in a suitably defined derivative expansion. We also show that interesting spin-transport effects appear at second order in the derivative expansion. Namely, we show that the fluid's rotation polarizes its spin. This is the hydrodynamic manifestation of the Barnett effect and provides us with an example of hydrodynamic spintronics. To conclude this thesis, we discuss several possible extensions of our research, as well as proposals for research in related directions.}, subject = {Hydrodynamics}, language = {en} } @article{KochereshkoDurnevBesombesetal.2016, author = {Kochereshko, Vladimir P. and Durnev, Mikhail V. and Besombes, Lucien and Mariette, Henri and Sapega, Victor F. and Askitopoulos, Alexis and Savenko, Ivan G. and Liew, Timothy C. H. and Shelykh, Ivan A. and Platonov, Alexey V. and Tsintzos, Simeon I. and Hatzopoulos, Z. and Savvidis, Pavlos G. and Kalevich, Vladimir K. and Afanasiev, Mikhail M. and Lukoshkin, Vladimir A. and Schneider, Christian and Amthor, Matthias and Metzger, Christian and Kamp, Martin and Hoefling, Sven and Lagoudakis, Pavlos and Kavokin, Alexey}, title = {Lasing in Bose-Fermi mixtures}, series = {Scientific Reports}, volume = {6}, journal = {Scientific Reports}, number = {20091}, doi = {10.1038/srep20091}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-168152}, year = {2016}, abstract = {Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also been developed based on Bose-Einstein condensates of exciton-polaritons in semiconductor microcavities. These electrically neutral bosons coexist with charged electrons and holes. In the presence of magnetic fields, the charged particles are bound to their cyclotron orbits, while the neutral exciton-polaritons move freely. We demonstrate how magnetic fields affect dramatically the phase diagram of mixed Bose-Fermi systems, switching between fermionic lasing, incoherent emission and bosonic lasing regimes in planar and pillar microcavities with optical and electrical pumping. We collected and analyzed the data taken on pillar and planar microcavity structures at continuous wave and pulsed optical excitation as well as injecting electrons and holes electronically. Our results evidence the transition from a Bose gas to a Fermi liquid mediated by magnetic fields and light-matter coupling.}, language = {en} } @article{WyborskiPodemskiWrońskietal.2022, author = {Wyborski, Paweł and Podemski, Paweł and Wroński, Piotr Andrzej and Jabeen, Fauzia and H{\"o}fling, Sven and Sęk, Grzegorz}, title = {Electronic and optical properties of InAs QDs grown by MBE on InGaAs metamorphic buffer}, series = {Materials}, volume = {15}, journal = {Materials}, number = {3}, issn = {1996-1944}, doi = {10.3390/ma15031071}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-297037}, year = {2022}, abstract = {We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.}, language = {en} } @article{AdrianMartinezAlbertAndreetal.2016, author = {Adri{\´a}n-Mart{\´i}nez, S. and Albert, A. and Andr{\´e}, M. and Anton, G. and Ardid, M. and Aubert, J.-J. and Avgitas, T. and Baret, B. and Barrios-Mart{\´i}, J. and Basa, S. and Bertin, V. and Biagi, S. and Bormuth, R. and Bouwhuis, M.C. and Bruijn, R. and Brunner, J. and Busto, J. and Capone, A. and Caramete, L. and Carr, J. and Celli, S. and Chiarusi, T. and Circella, M. and Coleiro, A. and Coniglione, R. and Costantini, H. and Coyle, P. and Creusot, A. and Deschamps, A. and De Bonis, G. and Distefano, C. and Donzaud, C. and Dornic, D. and Drouhin, D. and Eberl, T. and El Bojaddaini, I. and Els{\"a}sser, D. and Enzenh{\"o}fer, A. and Fehn, K. and Felis, I. and Fusco, L.A. and Galat{\`a}, S. and Gay, P. and Geißels{\"o}der, S. and Geyer, K. and Giordano, V. and Gleixner, A. and Glotin, H. and Gracia-Ruiz, R. and Graf, K. and Hallmann, S. and van Haren, H. and Heijboer, A.J. and Hello, Y. and Hern{\´a}ndez-Rey, J.J. and H{\"o}ßl, J. and Hofest{\"a}dt, J. and Hugon, C. and Illuminati, G. and James, C.W. and de Jong, M. and Jongen, M. and Kadler, M. and Kalekin, O. and Katz, U. and Kießling, D. and Kouchner, A. and Kreter, M. and Kreykenbohm, I. and Kulikovskiy, V. and Lachaud, C. and Lahmann, R. and Lef{\`e}vre, D. and Leonora, E. and Loucatos, S. and Marcelin, M. and Margiotta, A. and Marinelli, A. and Mart{\´i}nez-Mora, J.A. and Mathieu, A. and Melis, K. and Michael, T. and Migliozzi, P. and Moussa, A. and Mueller, C. and Nezri, E. and Pavalas, G.E. and Pellegrino, C. and Perrina, C. and Piattelli, P. and Popa, V. and Pradier, T. and Racca, C. and Riccobene, G. and Roensch, K. and Salda{\~n}a, M. and Samtleben, D.F.E. and S{\´a}nchez-Losa, A. and Sanguineti, M. and Sapienza, P. and Schnabel, J. and Sch{\"u}ssler, F. and Seitz, T. and Sieger, C. and Spurio, M. and Stolarczyk, Th. and Taiuti, M. and T{\"o}nnis, C. and Trovato, A. and Tselengidou, M. and Turpin, D. and Vallage, B. and Vall{\´e}e, C. and Van Elewyck, V. and Vivolo, D. and Wagner, S. and Wilms, J. and Zornoza, J.D. and Z{\´u}{\~n}iga, J.}, title = {Limits on dark matter annihilation in the sun using the ANTARES neutrino telescope}, series = {Physics Letters B}, volume = {759}, journal = {Physics Letters B}, doi = {10.1016/j.physletb.2016.05.019}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166642}, pages = {69-74}, year = {2016}, abstract = {A search for muon neutrinos originating from dark matter annihilations in the Sun is performed using the data recorded by the ANTARES neutrino telescope from 2007 to 2012. In order to obtain the best possible sensitivities to dark matter signals, an optimisation of the event selection criteria is performed taking into account the background of atmospheric muons, atmospheric neutrinos and the energy spectra of the expected neutrino signals. No significant excess over the background is observed and 90\% C.L. upper limits on the neutrino flux, the spin-dependent and spin-independent WIMP-nucleon cross-sections are derived for WIMP masses ranging from 50 GeV to 5 TeV for the annihilation channels WIMP + WIMP→ b\(\overline{b}\), W\(^{+}\)W\(^{-}\) and τ\(^{+}\)τ\(^{-}\).}, language = {en} } @article{KurzKampfBuschleetal.2016, author = {Kurz, Felix T. and Kampf, Thomas and Buschle, Lukas R. and Schlemmer, Heinz-Peter and Bendszus, Martin and Heiland, Sabine and Ziener, Christian H.}, title = {Generalized moment analysis of magnetic field correlations for accumulations of spherical and cylindrical magnetic perturbers}, series = {Frontiers in Physics}, volume = {4}, journal = {Frontiers in Physics}, issn = {2296-424X}, doi = {10.3389/fphy.2016.00046}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-190604}, year = {2016}, abstract = {In biological tissue, an accumulation of similarly shaped objects with a susceptibility difference to the surrounding tissue generates a local distortion of the external magnetic field in magnetic resonance imaging. It induces stochastic field fluctuations that characteristically influence proton spin dephasing in the vicinity of these magnetic perturbers. The magnetic field correlation that is associated with such local magnetic field inhomogeneities can be expressed in the form of a dynamic frequency autocorrelation function that is related to the time evolution of the measured magnetization. Here, an eigenfunction expansion for two simple magnetic perturber shapes, that of spheres and cylinders, is considered for restricted spin diffusion in a simple model geometry. Then, the concept of generalized moment analysis, an approximation technique that is applied in the study of (non-)reactive processes that involve Brownian motion, allows deriving analytical expressions of the correlation function for different exponential decay forms. Results for the biexponential decay for both spherical and cylindrical magnetized objects are derived and compared with the frequently used (less accurate) monoexponential decay forms. They are in asymptotic agreement with the numerically exact value of the correlation function for long and short times.}, language = {en} } @article{AdrianMartinezAlbertAndreetal.2016, author = {Adri{\´a}n-Mart{\´i}nez, S. and Albert, A. and Andr{\´e}, M. and Anghinolfi, M. and Anton, G. and Ardid, M. and Aubert, J.-J. and Avgitas, T. and Baret, B. and Barrios-Mart{\´i}, J. and Basa, S. and Bertin, V. and Biagi, S. and Bormuth, R. and Bouwhuis, M.C. and Bruijn, R. and Brunner, J. and Busto, J. and Capone, A. and Caramete, L. and Carr, J. and Celli, S. and Chiarusi, T. and Circella, M. and Coleiro, A. and Coniglione, R. and Constantini, H. and Coyle, P. and Creusot, A. and Deschamps, A. and De Bonis, G. and Distefano, C. and Donzaud, C. and Dornic, D. and Drouhin, D. and Eberl, T. and El Bojaddaini, I. and Els{\"a}sser, D. and Enzenh{\"o}fer, A. and Fehn, K. and Felis, I. and Fusco, L.A. and Galat{\`a}, S. and Gay, P. and Geißels{\"o}der, S. and Geyer, K. and Giordano, V. and Gleixner, A. and Glotin, H. and Gracia-Ruiz, R. and Graf, K. and Hallmann, S. and van Haren, H. and Heijboer, A.J. and Hello, Y. and Hern{\´a}ndez-Rey, J.J. and H{\"o}ßl, J. and Hofest{\"a}dt, J. and Hugon, C. and Illuminati, G. and James, C.W. and de Jong, M. and Kadler, M. and Kalekin, O. and Katz, U. and Kießling, D. and Kouchner, A. and Kreter, M. and Kreykenbohm, I. and Kulikovskiy, V. and Lachaud, C. and Lahmann, R. and Lef{\`e}vre, D. and Leonora, E. and Loucatos, S. and Marcelin, M. and Margiotta, A. and Marinelli, A. and Mart{\´i}nez-Mora, J.A. and Mathieu, A. and Michael, T. and Migliozzi, P. and Moussa, A. and Mueller, C. and Nezri, E. and Pavalas, G.E. and Pellegrino, C. and Perrina, C. and Piattelli, P. and Popa, V. and Pradier, T. and Racca, C. and Riccobene, G. and Roensch, K. and Salda{\~n}a, M. and Samtleben, D.F.E. and S{\´a}nchez-Losa, A. and Sanguineti, M. and Sapienza, P. and Schnabel, J. and Sch{\"u}ssler, F. and Seitz, T. and Sieger, C. and Spurio, M. and Stolarczyk, Th. and Taiuti, M. and Trovato, A. and Tselengidou, M. and Turpin, D. and T{\"o}nnis, C. and Vallage, B. and Vall{\´e}e, C. and Van Elewyck, V. and Visser, E. and Vivolo, D. and Wagner, S. and Wilms, J. and Zornoza, J.D. and Z{\´u}{\~n}iga, J.}, title = {Constraints on the neutrino emission from the Galactic Ridge with the ANTARES telescope}, series = {Physics Letters B}, volume = {760}, journal = {Physics Letters B}, doi = {10.1016/j.physletb.2016.06.051}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166608}, pages = {143-148}, year = {2016}, abstract = {A highly significant excess of high-energy astrophysical neutrinos has been reported by the IceCube Collaboration. Some features of the energy and declination distributions of IceCube events hint at a North/South asymmetry of the neutrino flux. This could be due to the presence of the bulk of our Galaxy in the Southern hemisphere. The ANTARES neutrino telescope, located in the Mediterranean Sea, has been taking data since 2007. It offers the best sensitivity to muon neutrinos produced by galactic cosmic ray interactions in this region of the sky. In this letter a search for an extended neutrino flux from the Galactic Ridge region is presented. Different models of neutrino production by cosmic ray propagation are tested. No excess of events is observed and upper limits for different neutrino flux spectral indices Γ are set. For Γ=2.4 the 90\% confidence level flux upper limit at 100 TeV for one neutrino flavour corresponds to Φ\(^{1f}_{0}\) (100 TeV) = 2.0 · 10\(^{-17}\) GeV\(^{-1}\) cm\(^{-2}\)s\(^{-1}\)sr\(^{-1}\). Under this assumption, at most two events of the IceCube cosmic candidates can originate from the Galactic Ridge. A simple power-law extrapolation of the Fermi-LAT flux to account for IceCube High Energy Starting Events is excluded at 90\% confidence level.}, language = {en} } @phdthesis{Pfister2019, author = {Pfister, Julian}, title = {Beschleunigte Magnetresonanz-Relaxographie}, doi = {10.25972/OPUS-18157}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181578}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {Ziel dieser Arbeit ist es, die quantitative MRT in den Fokus zu r{\"u}cken. In den letzten Jahren hat sich auf diesem Forschungsgebiet viel weiterentwickelt und es wurden verschiedenste Sequenzen und Methoden vorgestellt, um insbesondere Relaxationszeitparameter quantitativ in kurzer Zeit zu messen. Steady-State-Sequenzen eignen sich besonders f{\"u}r diese Thematik, da sie kurze Messzeiten ben{\"o}tigen und dar{\"u}ber hinaus ein relativ hohes SNR besitzen. Speziell die IR TrueFISP-Sequenz bietet f{\"u}r die Parameterquantifizierung viel Potential. Urspr{\"u}nglich wurde diese Sequenz an der Universit{\"a}t W{\"u}rzburg zur simultanen Messung von T1- und T2-Relaxationszeiten vorgestellt und hinsichtlich der Zeiteffizienz weiterentwickelt. In dieser Arbeit wurde ein neuartiger iterativer Rekonstruktionsansatz f{\"u}r die IR TrueFISP-Sequenz entwickelt, der auf einer Hauptkomponentenanalyse (PCA) basiert und sich die glatten Signalverl{\"a}ufe zu Nutze macht. Aufgrund der hohen Zeitaufl{\"o}sung dieser Rekonstruktionstechnik werden dabei auch Gewebekomponenten mit kurzen Relaxationszeiten detektierbar. Weiterhin bewahrt der Rekonstruktionsansatz Informationen mehrerer Gewebekomponenten innerhalb eines Voxels und erm{\"o}glicht damit eine relaxographische Untersuchung. Insbesondere beim Menschen f{\"u}hren der Partialvolumeneffekt und die Mikrostruktur des Gewebes zu Signalverl{\"a}ufen, die ein multi-exponentielles Signal liefern. Die MR-Relaxographie, also die Darstellung von Relaxationszeitverteilungen innerhalb eines Voxels, stellt eine M{\"o}glichkeit dar, um die beteiligten Gewebekomponenten aus dem {\"u}berlagerten Signalverlauf zu extrahieren. Insgesamt bilden die optimierte Relaxometrie mit der M{\"o}glichkeit der analytischen Korrektur von Magnetfeldinhomogenit{\"a}ten und die beschleunigte Relaxographie die Hauptteile dieser Dissertation. Die Hauptkapitel werden im Folgenden noch einmal gesondert zusammengefasst. Die simultane Aufnahme der quantitativen T1- und T2-Parameter-Karten kann mit einem Goldenen-Winkel-basiertem radialen IR TrueFISP-Readout in ungef{\"a}hr 7 Sekunden pro Schicht erreicht werden. Die bisherige Rekonstruktionstechnik mit dem KWIC-Filter ist durch dessen breite Filter-Bandbreite und somit in der zeitlichen Aufl{\"o}sung limitiert. Besonders bei hohen r{\"a}umlichen Frequenzen wird eine sehr große Anzahl an Projektionen zusammengefasst um ein Bild zu generieren. Dies sorgt daf{\"u}r, dass Gewebekomponenten mit kurzer T1*-Relaxationszeit (z.B. Fett oder Myelin) nicht akkurat aufgel{\"o}st werden k{\"o}nnen. Um dieses Problem zu umgehen, wurde die T1* shuffling-Rekonstruktion entwickelt, die auf dem T2 Shuffling-Ansatz basiert. Diese Rekonstruktionstechnik macht sich die glatten Signalverl{\"a}ufe der IR TrueFISP-Sequenz zu Nutze und erm{\"o}glicht die Anwendung einer PCA. Die iterative Rekonstruktion sorgt daf{\"u}r, dass mit nur acht kombinierten Projektionen pro generiertem Bild eine merklich verbesserte tempor{\"a}re Aufl{\"o}sung erzielt werden kann. Ein Nachteil ist jedoch das st{\"a}rkere Rauschen in den ersten Bildern der Zeitserie bedingt durch die angewandte PCA. Dieses verst{\"a}rkte Rauschen {\"a}ußert sich in den leicht erh{\"o}hten Standardabweichungen in den berechneten Parameter-Karten. Jedoch ist der Mittelwert n{\"a}her an den Referenzwerten im Vergleich zu den Ergebnissen mit dem KWIC-Filter. Letztendlich kann man sagen, dass die Ergebnisse leicht verrauschter, aber exakter sind. Mittels zus{\"a}tzlichen Regularisierungstechniken oder Vorwissen bez{\"u}glich des Rauschlevels w{\"a}re es zudem noch m{\"o}glich, das SNR der ersten Bilder zu verbessern, um dadurch den beschriebenen Effekt zu verringern. Grunds{\"a}tzlich h{\"a}ngt die Genauigkeit von IR TrueFISP vom T1/T2-Verh{\"a}ltnis des betreffenden Gewebes und dem gew{\"a}hlten Flipwinkel ab. In dieser Arbeit wurde der Flipwinkel besonders f{\"u}r weiße und graue Masse im menschlichen Gehirn optimiert. Mit den verwendeten 35° wurde er außerdem etwas kleiner gew{\"a}hlt, um zudem Magnetisierungstransfereffekte zu minimieren. Mit diesen Einstellungen ist die Pr{\"a}zision vor allem f{\"u}r hohe T1- und niedrige T2-Werte sehr gut, wird jedoch insbesondere f{\"u}r h{\"o}here T2-Werte schlechter. Dies ist aber ein generelles Problem der IR TrueFISP-Sequenz und h{\"a}ngt nicht mit der entwickelten Rekonstruktionsmethode zusammen. Außerdem wurde im f{\"u}nften Kapitel eine Akquisitionstechnik vorgestellt, die eine 3D-Abdeckung der quantitativen Messungen des Gehirns in klinisch akzeptabler Zeit von unter 10 Minuten erzielt. Dies wird durch Einsatz der parallelen Bildgebung erreicht, da eine Kombination aus radialer Abtastung in der Schicht und kartesischer Aufnahme in Schichtrichtung (Stack-of-Stars) vorliegt. Ein großes Problem in der Steady-State-Sequenz (und somit auch bei IR TrueFISP) sind Magnetfeldinhomogenit{\"a}ten, die durch Suszeptibilit{\"a}tsunterschiede verschiedener Gewebe und/oder Inhomogenit{\"a}ten des Hauptmagnetfeldes hervorgerufen werden. Diese f{\"u}hren zu Signalausl{\"o}schungen und damit verbunden zu den beschriebenen Banding-Artefakten. Mithilfe der analytisch ermittelten Korrekturformeln ist es nun m{\"o}glich, die berechneten (T1,T2)-Wertepaare unter Ber{\"u}cksichtigung der tats{\"a}chlich auftretenden Off- Resonanzfrequenz f{\"u}r einen großen Bereich zu korrigieren. An den kritischen Stellen, an denen die Bandings auftreten, liefert jedoch auch diese Korrektur keine brauchbaren Ergebnisse. Grunds{\"a}tzlich ist es f{\"u}r die Genauigkeit der Ergebnisse stets zu empfehlen, die Flipwinkel- und B0-Karte zus{\"a}tzlich mit aufzunehmen, um diese Parameter f{\"u}r die quantitative Auswertung exakt zu kennen. Mit den beschriebenen Methoden aus Kapitel 6 k{\"o}nnte es prinzipiell auch m{\"o}glich sein, die Off-Resonanzfrequenz aus dem Signalverlauf zu ermitteln und auf die zus{\"a}tzliche Messung der B0-Karte zu verzichten. B0-{\"A}nderungen w{\"a}hrend der Messung, die von der Erw{\"a}rmung der passiven Shim-Elemente im MR-System hervorgerufen werden, sind kaum zu korrigieren. Ein stabiler Scanner ohne B0-Drift ist deshalb f{\"u}r quantitative Auswertungen erforderlich. Die erw{\"a}hnte Messzeit von 7 Sekunden pro Schicht garantiert, dass auch Gewebe mit l{\"a}ngeren Relaxationskomponenten ann{\"a}hernd im Steady-State sind, was wiederum f{\"u}r das Umkehren des Signals in den abklingenden Verlauf gegen Null und die anschließende Multikomponentenanalyse (vgl. Kapitel 7) notwendig ist. Mit der inversen Laplace- Transformation ist es innerhalb eines Voxels m{\"o}glich, Signalverl{\"a}ufe auf mehrere Komponenten hin zu untersuchen. Der urspr{\"u}nglich angenommene mono-exponentielle Verlauf wird durch ein multi-exponentielles Verhalten abgel{\"o}st, was vor allem in biologischem Gewebe eher der Wahrheit entspricht. Gewebe mit kurzen Relaxationskomponenten (T1* < 200 ms) sind klinisch relevant und mit T1* shuffling detektierbar. Vor allem Myelin innerhalb des Gehirns ist bei neurologischen Fragestellungen ein Indikator zur Diagnose im Fr{\"u}hstadium (z.B. f{\"u}r neurodegenerative Erkrankungen) und deshalb von besonderem Interesse. Die Integration {\"u}ber verschiedene T1*-Zeitbereiche im T1*-Spektrum erm{\"o}glicht dazu die Erstellung von Gewebekomponenten-Karten, mithilfe derer klinische Auswertungen sinnvoll w{\"a}ren. Die Erstellung dieser Karten ist prinzipiell m{\"o}glich und funktioniert f{\"u}r mittlere und lange Gewebekomponenten recht gut. Die klinisch relevanten kurzen Gewebekomponenten sind dagegen bei der radialen Aufnahme mit nur einem Schuss noch nicht befriedigend. Deshalb wurde die Aufnahmetechnik in eine quasi-zuf{\"a}llige kartesische Akquisition mit mehreren Sch{\"u}ssen weiterentwickelt. Die Ergebnisse wurden in Kapitel 7 vorgestellt und sind vielversprechend. Einzig die Messzeit sollte mit zus{\"a}tzlichen Beschleunigungen noch weiter verk{\"u}rzt und auf eine kartesische 3D-Akquisition erweitert werden. Die Beschr{\"a}nkung auf T1*-Spektren bei der Multikomponentenanalyse und die Tatsache, dass deren Amplitude von einer Kombination von S0 und Sstst abh{\"a}ngen, f{\"u}hren dazu, dass es nicht ohne Weiteres m{\"o}glich ist f{\"u}r einen einzelnen Gewebetyp an die T1- und T2-Information zu gelangen. In Kapitel 8 wurde gezeigt, dass dies mit einer zus{\"a}tzlichen Messung gelingen kann. Das finale Ergebnis dieser Messungen ohne und mit Inversion sind zweidimensionale Spektren, bei der f{\"u}r jede Gewebekomponente innerhalb eines Voxels der T1- und T2-Wert abgelesen werden kann. Wichtig hierbei ist die Tatsache, dass der verwendete Ansatz kein Vorwissen {\"u}ber die Anzahl der zu erwartenden Gewebekomponenten (Peaks) im Voxel voraussetzt. Auch bei dieser Methodik ist die Kenntnis {\"u}ber den tats{\"a}chlichen Flipwinkel von Bedeutung, da dieser in den Formeln zur Berechnung von T1 und T2 verwendet wird. Die Stabilit{\"a}t des B0-Feldes ist hier ebenso von enormer Bedeutung, da {\"A}nderungen zwischen den beiden Messungen zu einem unterschiedlichen Steady-State und somit zu Abweichungen bei den nachfolgenden Berechnungen f{\"u}hren, die auf den selben Steady-State-Wert ausgelegt sind. Zusammenfassend l{\"a}sst sich sagen, dass mit dieser Arbeit die Grundlagen f{\"u}r genauere und robustere quantitative Messungen mittels Steady-State-Sequenzen gelegt wurden. Es wurde gezeigt, dass sich Relaxationszeitspektren f{\"u}r jedes einzelne Voxel generieren lassen. Dadurch ist eine verbesserte Auswertung m{\"o}glich, um genauere Aussagen {\"u}ber die Zusammensetzung einer Probe (vor allem beim menschlichen Gewebe) treffen zu k{\"o}nnen. Zudem wurde die Theorie f{\"u}r ultraschnelle 2D-Relaxographie-Messungen vorgestellt. Erste"Proof of Principle"-Experimente zeigen, dass es m{\"o}glich ist, 2D-Relaxationszeitspektren in sehr kurzer Zeit zu messen und graphisch darzustellen. Diese Aufnahme- und Datenverarbeitungstechnik ist in dieser Form einmalig und in der Literatur kann bis dato keine schnellere Methode gefunden werden.}, subject = {Kernspintomographie}, language = {de} } @phdthesis{Suchomel2022, author = {Suchomel, Holger Maximilian}, title = {Entwicklung elektrooptischer Bauteile auf der Basis von Exziton-Polaritonen in Halbleiter-Mikroresonatoren}, doi = {10.25972/OPUS-27163}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-271630}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Exziton-Polaritonen (Polaritonen), hybride Quasiteilchen, die durch die starke Kopplung von Quantenfilm-Exzitonen mit Kavit{\"a}tsphotonen entstehen, stellen auf Grund ihrer vielseitigen und kontrollierbaren Eigenschaften einen vielversprechenden Kandidaten f{\"u}r die Entwicklung einer neuen Generation von nichtlinearen und integrierten elektrooptischen Bauteilen dar. Die vorliegende Arbeit besch{\"a}ftigt sich mit der Entwicklung und Untersuchung kompakter elektrooptischer Bauelemente auf der Basis von Exziton-Polaritonen in Halbleitermikrokavit{\"a}ten. Als erstes wird die Implementierung einer elektrisch angeregten, oberfl{\"a}chenemittierenden Polariton-Laserdiode vorgestellt, die ohne ein externes Magnetfeld arbeiten kann. Daf{\"u}r wird der Schichtaufbau, der Q-Faktor, das Dotierprofil und die RabiAufspaltung der Polariton-Laserdiode optimiert. Der Q-Faktor des finalen Aufbaus bel{\"a}uft sich auf Q ~ 16.000, w{\"a}hrend die Rabi-Aufspaltung im Bereich von ~ 11,0 meV liegt. Darauf aufbauend werden Signaturen der Polariton-Kondensation unter elektrischer Anregung, wie ein nichtlinearer Anstieg der Intensit{\"a}t, die Reduktion der Linienbreite und eine fortgesetzte Verschiebung der Emission zu h{\"o}heren Energien oberhalb der ersten Schwelle, demonstriert. Ferner werden die Koh{\"a}renzeigenschaften des Polariton-Kondensats mittels Interferenzspektroskopie untersucht. Basierend auf den optimierten Halbleiter-Mikroresonatoren wird eine Kontaktplattform f{\"u}r die elektrische Anregung ein- und zweidimensionaler Gitterstrukturen entwickelt. Dazu wird die Bandstrukturbildung eines Quadrat- und Graphen-Gitters unter elektrischer Anregung im linearen Regime untersucht und mit den Ergebnissen der optischen Charakterisierung verglichen. Die erhaltenen Dispersionen lassen sich durch das zugeh{\"o}rige Tight-Binding-Modell beschreiben. Ferner wird auch eine elektrisch induzierte Nichtlinearit{\"a}t in der Emission demonstriert. Die untersuchte Laser-Mode liegt auf der H{\"o}he des unteren Flachbandes und an der Position der Γ-Punkte in der zweiten Brillouin-Zone. Die zugeh{\"o}rige Modenstruktur weist die erwartete Kagome-Symmetrie auf. Abschließend wird die Bandstrukturbildung eines SSH-Gitters mit eingebautem Defekt unter elektrischer Anregung untersucht und einige Eigenschaften des topologisch gesch{\"u}tzten Defektzustandes gezeigt. Dazu geh{\"o}rt vor allem die Ausbildung der lokalisierten Defektmode in der Mitte der S-Bandl{\"u}cke. Die erhaltenen Ergebnisse stellen einen wichtigen Schritt in der Realisierung eines elektrisch betriebenen topologischen Polariton-Lasers dar. Abschließend wird ein elektrooptisches Bauteil auf der Basis von Polaritonen in einem Mikrodrahtresonator vorgestellt, in dem sich die Propagation eines PolaritonKondensats mittels eines elektrostatischen Feldes kontrollieren l{\"a}sst. Das Funktionsprinzip des Polariton-Schalters beruht auf der Kombination einer elektrostatischen Potentialsenke unterhalb des Kontaktes und der damit verbundenen erh{\"o}hten ExzitonIonisationsrate. Der Schaltvorgang wird sowohl qualitativ als auch quantitativ analysiert und die Erhaltenen Ergebnisse durch die Modellierung des Systems {\"u}ber die GrossPitaevskii-Gleichung beschrieben. Zus{\"a}tzlich wird ein negativer differentieller Widerstand und ein bistabiles Verhalten in der Strom-Spannungs-Charakteristik in Abh{\"a}ngigkeit von der Ladungstr{\"a}gerdichte im Kontaktbereich beobachtet. Dieses Verhalten wird auf gegenseitig konkurrierende Kondensats-Zust{\"a}nde innerhalb der Potentialsenke und deren Besetzung und damit direkt auf den r{\"a}umlichen Freiheitsgrad der PolaritonZust{\"a}nde zur{\"u}ckgef{\"u}hrt.}, subject = {Drei-F{\"u}nf-Halbleiter}, language = {de} } @phdthesis{Wilfert2019, author = {Wilfert, Stefan}, title = {Rastertunnelmikroskopische und -spektroskopische Untersuchung von Supraleitern und topologischen Supraleitern}, doi = {10.25972/OPUS-18059}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-180597}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {Quantencomputer k{\"o}nnen manche Probleme deutlich effizienter l{\"o}sen als klassische Rechner. Bisherige Umsetzungen leiden jedoch an einer zu geringen Dekoh{\"a}renzzeit, weshalb die Lebenszeit der Quantenzust{\"a}nde einen limitierenden Faktor darstellt. Topologisch gesch{\"u}tzte Anregungen, wie Majorana-Fermionen, k{\"o}nnten hingegen dieses Hindernis {\"u}berwinden. Diese lassen sich beispielsweise in topologischen Supraleitern realisieren. Bis zum jetzigen Zeitpunkt existieren nur wenige Materialien, die dieses Ph{\"a}nomen aufweisen. Daher ist das Verst{\"a}ndnis der elektronischen Eigenschaften f{\"u}r solche Verbindungen von großer Bedeutung. In dieser Dissertation wird die Koexistenz von Supraleitung an der Probenoberfl{\"a}che und topologischem Oberfl{\"a}chenzustand (engl. topological surface state, TSS) auf potentiellen topologischen Supraleitern {\"u}berpr{\"u}ft. Diese beiden Bedingungen sind essentiell zur Ausbildung von topologischer Supraleitung in zeitumkehrgesch{\"u}tzten Systemen. Hierzu wird mittels Landaulevelspektroskopie und Quasiteilcheninterferenz das Vorhandensein des TSS am Ferminiveau auf Tl\$_{x}\$Bi\$_{2}\$Te\$_{3}\$ und Nb\$_{x}\$Bi\$_{2}\$Se\$_{3}\$ verifiziert, die mittels Transportmessungen als supraleitend identifiziert wurden. Anschließend folgen hochaufgel{\"o}ste Spektroskopien an der Fermienergie, um die supraleitenden Eigenschaften zu analysieren. Zur Interpretation der analysierten Eigenschaften wird zu Beginn der Ni-haltige Schwere-Fermion-Supraleiter TlNi\$_{2}\$Se\$_{2}\$ untersucht, der eine vergleichbare {\"U}bergangstemperatur besitzt. Anhand diesem werden die g{\"a}ngigen Messmethoden der Rastertunnelmikroskopie und -spektroskopie f{\"u}r supraleitende Proben vorgestellt und die Leistungsf{\"a}higkeit der Messapparatur demonstriert. Im Einklang mit der Literatur zeigt sich ein \$s\$-Wellencharakter des Paarungsmechanismus sowie die Formation eines f{\"u}r Typ~II-Supraleiter typischen Abrikosov-Gitters in schwachen externen Magnetfeldern. Im folgenden Teil werden die potentiellen topologischen Supraleiter Tl\$_{x}\$Bi\$_{2}\$Te\$_{3}\$ und Nb\$_{x}\$Bi\$_{2}\$Se\$_{3}\$ begutachtet, f{\"u}r die eindeutig ein TSS best{\"a}tigt wird. Allerdings weisen beide Materialien keine Oberfl{\"a}chensupraleitung auf, was vermutlich durch eine Entkopplung der Oberfl{\"a}che vom Volumen durch Bandverbiegung zu erkl{\"a}ren ist. Unbeabsichtigte Kollisionen der Spitze mit der Probe f{\"u}hren jedoch zu supraleitenden Spitzen, die wesentlich erh{\"o}hte Werte f{\"u}r die kritische Temperatur und das kritische Feld zeigen. Der letzte Abschnitt widmet sich dem supraleitenden Substrat Nb(110), f{\"u}r den der Reinigungsprozess erl{\"a}utert wird. Hierbei sind kurze Heizschritte bis nahe des Schmelzpunktes n{\"o}tig, um die bei Umgebungsbedingungen entstehende Sauerstoffrekonstruktion effektiv zu entfernen. Des Weiteren werden die elektronischen Eigenschaften untersucht, die eine Oberfl{\"a}chenresonanz zum Vorschein bringen. Hochaufgel{\"o}ste Messungen lassen eine durch die BCS-Theorie gut repr{\"a}sentierte Struktur der supraleitenden Energiel{\"u}cke erkennen. Magnetfeldabh{\"a}ngige Experimente offenbaren zudem eine mit der Kristallstruktur vereinbare Anisotropie des Paarungspotentials. Mit diesen Erkenntnissen kann Nb(110) zuk{\"u}nftig als Ausgang f{\"u}r das Wachstum von topologischen Supraleitern herangezogen werden.}, subject = {Supraleitung}, language = {de} } @article{HaackHauckKlingenbergetal.2021, author = {Haack, J. and Hauck, C. and Klingenberg, C. and Pirner, M. and Warnecke, S.}, title = {A Consistent BGK Model with Velocity-Dependent Collision Frequency for Gas Mixtures}, series = {Journal of Statistical Physics}, volume = {184}, journal = {Journal of Statistical Physics}, number = {3}, issn = {1572-9613}, doi = {10.1007/s10955-021-02821-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-269146}, pages = {31}, year = {2021}, abstract = {We derive a multi-species BGK model with velocity-dependent collision frequency for a non-reactive, multi-component gas mixture. The model is derived by minimizing a weighted entropy under the constraint that the number of particles of each species, total momentum, and total energy are conserved. We prove that this minimization problem admits a unique solution for very general collision frequencies. Moreover, we prove that the model satisfies an H-Theorem and characterize the form of equilibrium.}, language = {en} } @article{BiedermannBraeuerDenneretal.2017, author = {Biedermann, Benedikt and Br{\"a}uer, Stephan and Denner, Ansgar and Pellen, Mathieu and Schumann, Steffen and Thompson, Jennifer M.}, title = {Automation of NLO QCD and EW corrections with SHERPA and RECOLA}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, number = {492}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-170615}, year = {2017}, abstract = {This publication presents the combination of the one-loop matrix-element generator Recola with the multipurpose Monte Carlo program Sherpa. Since both programs are highly automated, the resulting Sherpa +Recola framework allows for the computation of - in principle - any Standard Model process at both NLO QCD and EW accuracy. To illustrate this, three representative LHC processes have been computed at NLO QCD and EW: vector-boson production in association with jets, off-shell Z-boson pair production, and the production of a top-quark pair in association with a Higgs boson. In addition to fixed-order computations, when considering QCD corrections, all functionalities of Sherpa, i.e. particle decays, QCD parton showers, hadronisation, underlying events, etc. can be used in combination with Recola. This is demonstrated by the merging and matching of one-loop QCD matrix elements for Drell-Yan production in association with jets to the parton shower. The implementation is fully automatised, thus making it a perfect tool for both experimentalists and theorists who want to use state-of-the-art predictions at NLO accuracy.}, language = {en} } @phdthesis{Namal2018, author = {Namal, Imge}, title = {Fabrication and Optical and Electronic Characterization of Conjugated Polymer-Stabilized Semiconducting Single-Wall Carbon Nanotubes in Dispersions and Thin Films}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-162393}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {In order to shrink the size of semiconductor devices and improve their efficiency at the same time, silicon-based semiconductor devices have been engineered, until the material almost reaches its performance limits. As the candidate to be used next in semiconducting devices, single-wall carbon nanotubes show a great potential due to their promise of increased device efficiency and their high charge carrier mobilities in the nanometer size active areas. However, there are material based problems to overcome in order to imply SWNTs in the semiconductor devices. SWNTs tend to aggregate in bundles and it is not trivial to obtain an electronically or chirally homogeneous SWNT dispersion and when it is done, a homogeneous thin film needs to be produced with a technique that is practical, easy and scalable. This work was aimed to solve both of these problems. In the first part of this study, six different polymers, containing fluorene or carbazole as the rigid part and bipyridine, bithiophene or biphenyl as the accompanying copolymer unit, were used to selectively disperse semiconducting SWNTs. With the data obtained from absorption and photoluminescence spectroscopy of the corresponding dispersions, it was found out that the rigid part of the copolymer plays a primary role in determining its dispersion efficiency and electronic sorting ability. Within the two tested units, carbazole has a higher π electron density. Due to increased π-π interactions, carbazole containing copolymers have higher dispersion efficiency. However, the electronic sorting ability of fluorene containing polymers is superior. Chiral selection of the polymers in the dispersion is not directly foreseeable from the selection of backbone units. At the end, obtaining a monochiral dispersion is found to be highly dependent on the used raw material in combination to the preferred polymer. Next, one of the best performing polymers due to high chirality enrichment and electronic sorting ability was chosen in order to disperse SWNTs. Thin films of varying thickness between 18 ± 5 to 755o±o5 nm were prepared using vacuum filtration wet transfer method in order to analyze them optically and electronically. The scalability and efficiency of the integrated thin film production method were shown using optical, topographical and electronic measurements. The relative photoluminescence quantum yield of the radiative decay from the SWNT thin films was found to be constant for the thickness scale. Constant roughness on the film surface and linearly increasing concentration of SWNTs were also supporting the scalability of this thin film production method. Electronic measurements on bottom gate top contact transistors have shown an increasing charge carrier mobility for linear and saturation regimes. This was caused by the missing normalization of the mobility for the thickness of the active layer. This emphasizes the importance of considering this dimension for comparison of different field effect transistor mobilities.}, subject = {Feldeffekttransistor}, language = {en} } @phdthesis{Kreutner2018, author = {Kreutner, Jakob}, title = {Charakterisierung des Knochens und seiner Mikrostruktur mit hochaufl{\"o}sender 3D-MRT}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-168858}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Neue Therapieans{\"a}tze durch Tissue Engineering erfordern gleichzeitig angepasste Diagnosem{\"o}glichkeiten und nicht-invasive Erfolgskontrollen. Speziell die 3D-MR-Bildgebung ist ein vielversprechendes Instrument, um Parameter mit hoher r{\"a}umlicher Pr{\"a}zision zu quantifizieren. Vor diesem Hintergrund wurden im Rahmen dieser Arbeit neue Ans{\"a}tze f{\"u}r die hochaufl{\"o}sende 3D-MRT in vivo entwickelt und deren Eignung im Bereich des Tissue Engineerings gezeigt. Welchen Vorteil die Quantifizierung von Parametern bietet, konnte im Rahmen einer pr{\"a}-klinischen Studie an einem Modell der H{\"u}ftkopfnekrose gezeigt werden. Der Therapieverlauf wurde zu verschiedenen Zeitpunkten kontrolliert. Trotz der niedrigen r{\"a}umlichen Aufl{\"o}sung, konnten durch eine systematische Auswertung der Signalintensit{\"a}ten von T1- und T2-FS-gewichteten Aufnahmen R{\"u}ckschl{\"u}sse {\"u}ber Ver{\"a}nderungen in der Mikrostruktur gezogen werden, die dar{\"u}ber hinaus in guter {\"U}bereinstimmung mit Ergebnissen von ex vivo µCT-Aufnahmen waren. Dort konnte eine Verdickung der Trabekelstruktur nachgewiesen werden, welche sehr gut mit einer Signalabnahme in den T1-gewichteten Aufnahmen korrelierte. Die radiale Auswertung der Daten erlaubte dabei eine komprimierte Darstellung der Ergebnisse. Dadurch wurde eine effiziente Auswertung der umfangreichen Daten (verschiedene Tiere an mehreren Zeitpunkten mit einer Vielzahl an Einzelaufnahmen) erm{\"o}glicht und eine unabh{\"a}ngige Bewertung erreicht. Um die Limitationen der begrenzten Aufl{\"o}sung von 2D-Multi-Schichtaufnahmen aufzuheben, wurden neue Ans{\"a}tze f{\"u}r eine hochaufgel{\"o}ste 3D-Aufnahme entwickelt. Hierf{\"u}r wurden Spin-Echo-basierte Sequenzen gew{\"a}hlt, da diese eine genauere Abbildung der Knochenmikrostruktur erlauben als Gradienten-Echo-basierte Methoden. Zum einen wurde eine eigene 3D-FLASE-Sequenz entwickelt und zum anderen eine modifizierte 3D-TSE-Sequenz. Damit an Patienten Aufnahmen bei klinischer Feldst{\"a}rke von 1,5 T mit einer hohen r{\"a}umlichen Aufl{\"o}sung innerhalb einer vertretbaren Zeit erzielt werden k{\"o}nnen, muss eine schnelle und signalstarke Sequenz verwendet werden. Eine theoretische Betrachtung bescheinigte der TSE-Sequenz eine um 25 \% h{\"o}here Signaleffizienz verglichen mit einer FLASE-Sequenz mit identischer Messzeit. Dieser Unterschied konnte auch im Experiment nachgewiesen werden. Ein in vivo Vergleich der beiden Sequenzen am Schienbein zeigte eine vergleichbare Darstellung der Spongiosa mit einer Aufl{\"o}sung von 160 × 160 × 400 µm. F{\"u}r die Bildgebung des H{\"u}ftkopfs mit der neuen Sequenz waren jedoch aufgrund der unterschiedlichen Anatomie weitere Modifikationen notwendig. Um l{\"a}ngere Messzeiten durch ein unn{\"o}tig großes Field-of-View zu vermeiden, mussten Einfaltungsartefakte unterdr{\"u}ckt werden. Dies wurde durch die orthogonale Anwendung der Anregungs- und Refokussierungspulse in der TSE-Sequenz effizient gel{\"o}st. Technisch bedingt konnte jedoch nicht eine vergleichbare Aufl{\"o}sung wie am Schienbein realisiert werden. Der Vorteil der 3D-Bildgebung, dass Schichtdicken von deutlich weniger als 1 mm erreicht werden k{\"o}nnen, konnte jedoch erfolgreich auf den Unterkiefer {\"u}bertragen werden. Der dort verlaufende Nervus Mandibularis ist dabei eine wichtige Struktur, deren Verlauf im Vorfeld von verschiedenen operativen Eingriffen bekannt sein muss. Er ist durch eine d{\"u}nne kn{\"o}cherne Wand vom umgebenden Gewebe getrennt. Im Vergleich mit einer 3D-VIBE-Sequenz zeigte die entwickelte 3D-TSE-Sequenz mit integrierter Unterdr{\"u}ckung von Einfaltungsartefakten eine {\"a}hnlich gute Lokalisierung des Nervenkanals {\"u}ber die gesamte L{\"a}nge der Struktur. Dies konnte in einer Studie an gesunden Probanden mit verschiedenen Beobachtern nachgewiesen werden. Durch die neue Aufnahmetechnik konnte dar{\"u}ber hinaus die Aufl{\"o}sung im Vergleich zu bisherigen Studien deutlich erh{\"o}ht werden, was insgesamt eine pr{\"a}zisere Lokalisierung des Nervenkanals erlaubt. Ein Baustein des Tissue Engineerings sind bio-resorbierbare Materialien, deren Abbau- und Einwachsverhalten noch untersucht werden muss, bevor diese f{\"u}r die klinische Anwendung zugelassen werden. Die durchgef{\"u}hrten in vitro µMR-Untersuchungen an Polymerscaffolds zeigten die reproduzierbare Quantifizierung der Porengr{\"o}ße und Wandst{\"a}rke. Dar{\"u}ber hinaus wurde eine inhomogene Verteilung der Strukturparameter beobachtet. Die Ergebnisse waren in guter {\"U}bereinstimmung mit µCT-Aufnahmen als Goldstandard. Unterschiedliche Varianten der Scaffolds konnten identifiziert werden. Dabei bewies sich die MR-Bildgebung als zuverl{\"a}ssige Alternative. Insgesamt zeigen die Ergebnisse dieser Arbeit, welche Vorteile und Anwendungsm{\"o}glichkeiten die 3D-MRT-Bildgebung bietet, und dass auch mit klinischer Feldst{\"a}rke in vivo Voxelgr{\"o}ßen im Submillimeterbereich f{\"u}r alle Raumrichtungen erreichbar sind. Die erzielten Verbesserungen in der r{\"a}umlichen Aufl{\"o}sung erh{\"o}hen die Genauigkeit der verschiedenen Anwendungen und erm{\"o}glichen eine bessere Identifikation von kleinen Abweichungen, was eine fr{\"u}here und zuverl{\"a}ssigere Diagnose f{\"u}r Patienten verspricht.}, subject = {Kernspintomografie}, language = {de} } @article{GrueneLondiGillettetal.2023, author = {Gr{\"u}ne, Jeannine and Londi, Giacomo and Gillett, Alexander J. and St{\"a}hly, Basil and Lulei, Sebastian and Kotova, Maria and Olivier, Yoann and Dyakonov, Vladimir and Sperlich, Andreas}, title = {Triplet Excitons and Associated Efficiency-Limiting Pathways in Organic Solar Cell Blends Based on (Non-) Halogenated PBDB-T and Y-Series}, series = {Advanced Functional Materials}, volume = {33}, journal = {Advanced Functional Materials}, number = {12}, doi = {10.1002/adfm.202212640}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312164}, year = {2023}, abstract = {The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non-fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20\%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low-lying states that are responsible for non-radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin-sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum-chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB-T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non-geminate hole back transfer and, in blends with halogenated donors, also by spin-orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies.}, language = {en} } @article{MotykaDyksikRyczkoetal.2016, author = {Motyka, M. and Dyksik, M. and Ryczko, K. and Weih, R. and Dallner, M. and H{\"o}fling, S. and Kamp, M. and Sęk, G. and Misiewicz, J.}, title = {Type-II quantum wells with tensile-strained GaAsSb layers for interband cascade lasers with tailored valence band mixing}, series = {Applied Physics Letters}, volume = {108}, journal = {Applied Physics Letters}, number = {10}, doi = {10.1063/1.4943193}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-189795}, year = {2016}, abstract = {Optical properties of modified type II W-shaped quantum wells have been investigated with the aim to be utilized in interband cascade lasers. The results show that introducing a tensely strained GaAsSb layer, instead of a commonly used compressively strained GaInSb, allows employing the active transition involving valence band states with a significant admixture of the light holes. Theoretical predictions of multiband k.p theory have been experimentally verified by using photoluminescence and polarization dependent photoreflectance measurements. These results open a pathway for practical realization of mid-infrared lasing devices with uncommon polarization properties including, for instance, polarization-independent midinfrared light emitters.}, language = {en} } @phdthesis{Schrauth2021, author = {Schrauth, Manuel}, title = {Critical Phenomena in Topologically Disordered Systems}, doi = {10.25972/OPUS-23499}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-234998}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {Clearly, in nature, but also in technological applications, complex systems built in an entirely ordered and regular fashion are the exception rather than the rule. In this thesis we explore how critical phenomena are influenced by quenched spatial randomness. Specifically, we consider physical systems undergoing a continuous phase transition in the presence of topological disorder, where the underlying structure, on which the system evolves, is given by a non-regular, discrete lattice. We therefore endeavour to achieve a thorough understanding of the interplay between collective dynamics and quenched randomness. According to the intriguing concept of universality, certain laws emerge from collectively behaving many-body systems at criticality, almost regardless of the precise microscopic realization of interactions in those systems. As a consequence, vastly different phenomena show striking similarities at their respective phase transitions. In this dissertation we pursue the question of whether the universal properties of critical phenomena are preserved when the system is subjected to topological perturbations. For this purpose, we perform numerical simulations of several prototypical systems of statistical physics which show a continuous phase transition. In particular, the equilibrium spin-1/2 Ising model and its generalizations represent -- among other applications -- fairly natural approaches to model magnetism in solids, whereas the non-equilibrium contact process serves as a toy model for percolation in porous media and epidemic spreading. Finally, the Manna sandpile model is strongly related to the concept of self-organized criticality, where a complex dynamic system reaches a critical state without fine-tuning of external variables. Our results reveal that the prevailing understanding of the influence of topological randomness on critical phenomena is insufficient. In particular, by considering very specific and newly developed lattice structures, we are able to show that -- contrary to the popular opinion -- spatial correlations in the number of interacting neighbours are not a key measure for predicting whether disorder ultimately alters the behaviour of a given critical system.}, subject = {Ising-Modell}, language = {en} } @phdthesis{Bendias2018, author = {Bendias, Michel Kalle}, title = {Quantum Spin Hall Effect - A new generation of microstructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-168214}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {The presented thesis summarizes the results from four and a half years of intense lithography development on (Cd,Hg)Te/HgTe/(Cd,Hg)Te quantum well structures. The effort was motivated by the unique properties of this topological insulator. Previous work from Molenkamp at al.\ has proven that the transport through such a 2D TI is carried by electrons with opposite spin, counter-propagating in 1D channels along the sample edge. However, up to this thesis, the length of quantized spin Hall channels has never been reported to exceed 4 µm. Therefore, the main focus was put on a reproducible and easy-to-handle fabrication process that reveals the intrinsic material parameters. Every single lithography step in macro as well as microscopic sample fabrication has been re-evaluated. In the Development, the process changes have been presented along SEM pictures, microgaphs and, whenever possible, measurement responses. We have proven the conventional ion milling etch method to damage the remaining mesa and result in drastically lower electron mobilities in samples of microscopic size. The novel KI:I2:HBr wet etch method for macro and microstructure mesa fabrication has been shown to leave the crystalline structure intact and result in unprecedented mobilities, as high as in macroscopic characterization Hall bars. Difficulties, such as an irregular etch start and slower etching of the conductive QW have been overcome by concentration, design and etch flow adaptations. In consideration of the diffusive regime, a frame around the EBL write field electrically decouples the structure mesa from the outside wafer. As the smallest structure, the frame is etched first and guarantees a non-different etching of the conductive layer during the redox reaction. A tube-pump method assures reproducible etch results with mesa heights below 300 nm. The PMMA etch mask is easy to strip and leaves a clean mesa with no redeposition. From the very first attempts, to the final etch process, the reader has been provided with the characteristics and design requirements necessary to enable the fabrication of nearly any mesa shape within an EBL write field of 200 µm. Magneto resistance measurement of feed-back samples have been presented along the development chronology of wet etch method and subsequent lithography steps. With increasing feature quality, more and more physics has been revealed enabling detailed evaluation of smallest disturbances. The following lithography improvements have been implemented. They represent a tool-box for high quality macro and microstructure fabrication on (CdHg)Te/HgTe of almost any kind. The optical positive resist ECI 3027 can be used as wet and as dry etch mask for structure sizes larger than 1 µm. It serves to etch mesa structures larger than the EBL write field. The double layer PMMA is used for ohmic contact fabrication within the EBL write field. Its thickness allows to first dry etch the (Cd,Hg)Te cap layer and then evaporate the AuGe contact, in situ and self-aligned. Because of an undercut, up to 300 nm can be metalized without any sidewalls after the lift-off. An edge channel mismatch within the contact leads can be avoided, if the ohmic contacts are designed to reach close to the sample and beneath the later gate electrode. The MIBK cleaning step prior to the gate application removes PMMA residuals and thereby improves gate and potential homogeneity. The novel low HfO2-ALD process enables insulator growth into optical and EBL lift-off masks of any resolvable shape. Directly metalized after the insulator growth, the self-aligned method results in thin and homogeneous gate electrode reproducibly withholding gate voltages to +-10 V. The optical negative resist ARN 4340 exhibits an undercut when developed. Usable as dry etch mask and lift-off resist, it enables an in-situ application of ohmic contacts first etching close to the QW, then metalizing AuGe. Up to 500 nm thickness, the undercut guarantees an a clean lift-off with no sidewalls. The undertaken efforts have led to micro Hall bar measurements with Hall plateaus and SdH-oszillations in up to now unseen levels of detail. The gap resistance of several micro Hall bars with a clear QSH signal have been presented in Quantum Spin Hall. The first to exhibit longitudinal resistances close to the expected h/2e2 since years, they reveal unprecedented details in features and characteristics. It has been shown that their protection against backscattering through time reversal symmetry is not as rigid as previously claimed. Values below and above 12.9 kΩ been explained, introducing backscattering within the Landauer-B{\"u}ttiker formalism of edge channel transport. Possible reasons have been discussed. Kondo, interaction and Rashba-backscattering arising from density inhomogeneities close to the edge are most plausible to explain features on and deviations from a quantized value. Interaction, tunneling and dephasing mechanisms as well as puddle size, density of states and Rashba Fields are gate voltage dependent. Therefore, features in the QSH signal are fingerprints of the characteristic potential landscape. Stable up to 11 K, two distinct but clear power laws have been found in the higher temperature dependence of the QSH in two samples. However, with ΔR = Tα, α = ¼ in one (QC0285) and α = 2 in the other (Q2745), none of the predicted dependencies could be confirmed. Whereas, the gap resistances of QC0285 remains QSH channel dominated up to 3.9 T and thereby confirmed the calculated lifting of the band inversion in magnetic field. The gate-dependent oscillating features in the QSH signal of Q2745 immediately increase in magnetic field. The distinct field dependencies allowed the assumption of two different dominant backscattering mechanisms. Resulting in undisturbed magneto transport and unprecedented QSH measurements The Novel Micro Hall Bar Process has proven to enable the fabrication of a new generation of microstructures.}, subject = {Quecksilbertellurid}, language = {en} } @article{MaCalvoWangetal.2015, author = {Ma, Eric Yue and Calvo, M. Reyes and Wang, Jing and Lian, Biao and M{\"u}hlbauer, Mathias and Br{\"u}ne, Christoph and Cui, Yong-Tao and Lai, Keji and Kundhikanjana, Worasom and Yang, Yongliang and Baenninger, Matthias and K{\"o}nig, Markus and Ames, Christopher and Buhmann, Hartmut and Leubner, Philipp and Molenkamp, Laurens W. and Zhang, Shou-Cheng and Goldhaber-Gordon, David and Kelly, Michael A. and Shen, Zhi-Xun}, title = {Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, number = {7252}, doi = {10.1038/ncomms8252}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143185}, year = {2015}, abstract = {The realization of quantum spin Hall effect in HgTe quantum wells is considered a milestone in the discovery of topological insulators. Quantum spin Hall states are predicted to allow current flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction yet to be experimentally verified is the breakdown of the edge conduction under broken time-reversal symmetry. Here we first establish a systematic framework for the magnetic field dependence of electrostatically gated quantum spin Hall devices. We then study edge conduction of an inverted quantum well device under broken time-reversal symmetry using microwave impedance microscopy, and compare our findings to a noninverted device. At zero magnetic field, only the inverted device shows clear edge conduction in its local conductivity profile, consistent with theory. Surprisingly, the edge conduction persists up to 9 T with little change. This indicates physics beyond simple quantum spin Hall model, including material-specific properties and possibly many-body effects.}, language = {en} } @article{DeaconWiedenmannBocquillonetal.2017, author = {Deacon, R. S. and Wiedenmann, J. and Bocquillon, E. and Dom{\´i}nguez, F. and Klapwijk, T. M. and Leubner, P. and Br{\"u}ne, C. and Hankiewicz, E. M. and Tarucha, S. and Ishibashi, K. and Buhmann, H. and Molenkamp, L. W.}, title = {Josephson Radiation from Gapless Andreev Bound States in HgTe-Based Topological Junctions}, series = {Physical Review X}, volume = {7}, journal = {Physical Review X}, number = {021011}, doi = {10.1103/PhysRevX.7.021011}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-170969}, year = {2017}, abstract = {Frequency analysis of the rf emission of oscillating Josephson supercurrent is a powerful passive way of probing properties of topological Josephson junctions. In particular, measurements of the Josephson emission enable the detection of topological gapless Andreev bound states that give rise to emission at half the Josephson frequency f\(_{J}\) rather than conventional emission at f\(_{J}\). Here, we report direct measurement of rf emission spectra on Josephson junctions made of HgTe-based gate-tunable topological weak links. The emission spectra exhibit a clear signal at half the Josephson frequency f\(_{J}\)/2. The linewidths of emission lines indicate a coherence time of 0.3-4 ns for the f\(_{J}\)/2 line, much shorter than for the f\(_{J}\) line (3-4 ns). These observations strongly point towards the presence of topological gapless Andreev bound states and pave the way for a future HgTe-based platform for topological quantum computation.}, language = {en} } @article{UllherrDiezZabler2022, author = {Ullherr, Maximilian and Diez, Matthias and Zabler, Simon}, title = {Robust image reconstruction strategy for multiscalar holotomography}, series = {Journal of Imaging}, volume = {8}, journal = {Journal of Imaging}, number = {2}, issn = {2313-433X}, doi = {10.3390/jimaging8020037}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-262112}, year = {2022}, abstract = {Holotomography is an extension of computed tomography where samples with low X-ray absorption can be investigated with higher contrast. In order to achieve this, the imaging system must yield an optical resolution of a few micrometers or less, which reduces the measurement area (field of view = FOV) to a few mm at most. If the sample size, however, exceeds the field of view (called local tomography or region of interest = ROI CT), filter problems arise during the CT reconstruction and phase retrieval in holotomography. In this paper, we will first investigate the practical impact of these filter problems and discuss approximate solutions. Secondly, we will investigate the effectiveness of a technique we call "multiscalar holotomography", where, in addition to the ROI CT, a lower resolution non-ROI CT measurement is recorded. This is used to avoid the filter problems while simultaneously reconstructing a larger part of the sample, albeit with a lower resolution in the additional area.}, language = {en} } @phdthesis{Du2019, author = {Du, Yiqiang}, title = {Gauge/Gravity Duality with Backreacting Background}, doi = {10.25972/OPUS-18786}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-187869}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {The topic of this thesis is generalizations of the Anti de Sitter/Conformal Field Theory (AdS/CFT) correspondence, often referred to as holography, and their application to models relevant for condensed matter physics. A particular virtue of AdS/CFT is to map strongly coupled quantum field theories, for which calculations are inherently difficult, to more tractable classical gravity theories. I use this approach to study the crossover between Bose-Einstein condensation (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superconductivity mechanism. I also study the phase transitions between the AdS black hole and AdS soliton spacetime in the presence of disorder. Moreover, I consider a holographic model of a spin impurity interacting with a strongly correlated electron gas, similar to the Kondo model. In AdS/CFT, the BEC/BCS crossover is modeled by a soliton configuration in the dual geometry and we study the BEC and BCS limits. The backreaction of the matter field on the background geometry is considered, which provides a new approach to study the BEC/BCS crossover. The behaviors of some physical quantities such as depletion of charge density under different strength of backreaction are presented and discussed. Moreover, the backreaction enables us to obtain the effective energy density of the soliton configurations, which together with the surface tension of the solitons leads to an argument for the occurrence of so called snake instability for dark solitons, i.e. for the solitons to form a vortex-like structures. Disordering strongly coupled and correlated quantum states of matter may lead to new insights into the physics of many body localized (MBL) strongly correlated states, which may occur in the presence of strong disorder. We are interested in potential insulator-metal transitions induced by disorder, and how disorder affects the Hawking-Page phase transition in AdS gravity in general. We introduce a metric ansatz and numerically construct the corresponding disordered AdS soliton and AdS black hole solutions, and discuss the calculation of the free energy in these states. In the Kondo effect, the rise in resistivity in metals with scarce magnetic impurities at low temperatures can be explained by the RG flow of the antiferromagnetic coupling between the impurity and conduction electrons in CFT. The generalizations to SU(N) in the large N limit make the treatment amenable to the holographic approach. We add a Maxwell term to a previously existing holographic model to study the conductivity of the itinerant electrons. Our goal is to find the log(T) behavior in the DC resistivity. In the probe limit, we introduce junction conditions to connect fields crossing the defect. We then consider backreactions, which give us a new metric ansatz and new junction conditions for the gauge fields.}, language = {en} } @phdthesis{Lundt2019, author = {Lundt, Nils}, title = {Strong light-matter coupling with 2D materials}, doi = {10.25972/OPUS-18733}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-187335}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {This publication is dedicated to investigate strong light-matter coupling with excitons in 2D materials. This work starts with an introduction to the fundamentals of excitons in 2D materials, microcavities and strong coupling in chapter 2. The experimental methods used in this work are explained in detail in chapter 3. Chapter 4 covers basic investigations that help to select appropriate materials and cavities for the following experiments. In chapter 5, results on the formation of exciton-polaritons in various materials and cavity designs are presented. Chapter 6 covers studies on the spin-valley properties of exciton-polaritons including effects such as valley polarization, valley coherence and valley-dependent polariton propagation. Finally, the formation of hybrid-polaritons and their condensation are presented in chapter 7.}, subject = {Exziton-Polariton}, language = {en} } @phdthesis{Gross2019, author = {Groß, Heiko}, title = {Controlling Light-Matter Interaction between Localized Surface Plasmons and Quantum Emitters}, doi = {10.25972/OPUS-19209}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-192097}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {Metal nanostructures have been known for a long time to exhibit optical resonances via localized surface plasmons. The high electric fields in close proximity to the metal surface have prospects to dramatically change the dynamics of electronic transitions, such as an enhanced spontaneous decay rate of a single emitter. However, there have been two major issues which impede advances in the experimental realization of enhanced light-matter interaction. (i) The fabrication of high-quality resonant structures requires state-of-the-art patterning techniques in combination with superior materials. (ii) The tiny extension of the optical near-field requires precise control of the single emitter with respect to the nanostructure. This work demonstrates a solution to these problems by combining scanning probe and optical confocal microscopy. Here, a novel type of scanning probe is introduced which features a tip composed of the edge of a single crystalline gold sheet. The patterning via focused ion beam milling makes it possible to introduce a plasmonic nanoresonator directly at the apex of the tip. Numerical simulations demonstrate that the optical properties of this kind of scanning probe are ideal to analyze light-matter interaction. Detailed experimental studies investigate the coupling mechanism between a localized plasmon and single colloidal quantum dots by dynamically changing coupling strength via their spatial separation. The results have shown that weak interaction affects the shape of the fluorescence spectrum as well as the polarization. For the best probes it has been found that it is possible to reach the strong coupling regime at the single emitter level at room temperature. The resulting analysis of the experimental data and the proposed theoretical models has revealed the differences between the established far-field coupling and near-field coupling. It has been found that the broad bandwidth of plasmonic resonances are able to establish coherent coupling to multiple transitions simultaneously giving rise to an enhanced effective coupling strength. It has also been found that the current model to numerically calculate the effective mode volume is inaccurate in case of mesoscopic emitters and strong coupling. Finally, light-matter interaction is investigated by the means of a quantum-dot-decorated microtubule which is traversing a localized nearfield by gliding on kinesin proteins. This biological transport mechanism allows the parallel probing of a meta-surface with nm-precision. The results that have been put forward throughout this work have shed new light on the understanding of plasmonic light-matter interaction and might trigger ideas on how to more efficiently combine the power of localized electric fields and novel excitonic materials.}, subject = {Plasmon}, language = {en} } @article{GottschollWagenhoeferKlimmeretal.2022, author = {Gottscholl, Andreas and Wagenh{\"o}fer, Maximilian and Klimmer, Manuel and Scherbel, Selina and Kasper, Christian and Baianov, Valentin and Astakhov, Georgy V. and Dyakonov, Vladimir and Sperlich, Andreas}, title = {Superradiance of spin defects in silicon carbide for maser applications}, series = {Frontiers in Photonics}, volume = {3}, journal = {Frontiers in Photonics}, issn = {2673-6853}, doi = {10.3389/fphot.2022.886354}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-284698}, year = {2022}, abstract = {Masers as telecommunication amplifiers have been known for decades, yet their application is strongly limited due to extreme operating conditions requiring vacuum techniques and cryogenic temperatures. Recently, a new generation of masers has been invented based on optically pumped spin states in pentacene and diamond. In this study, we pave the way for masers based on spin S = 3/2 silicon vacancy (V\(_{Si}\)) defects in silicon carbide (SiC) to overcome the microwave generation threshold and discuss the advantages of this highly developed spin hosting material. To achieve population inversion, we optically pump the V\(_{Si}\) into their m\(_S\) = ±1/2 spin sub-states and additionally tune the Zeeman energy splitting by applying an external magnetic field. In this way, the prerequisites for stimulated emission by means of resonant microwaves in the 10 GHz range are fulfilled. On the way to realising a maser, we were able to systematically solve a series of subtasks that improved the underlying relevant physical parameters of the SiC samples. Among others, we investigated the pump efficiency as a function of the optical excitation wavelength and the angle between the magnetic field and the defect symmetry axis in order to boost the population inversion factor, a key figure of merit for the targeted microwave oscillator. Furthermore, we developed a high-Q sapphire microwave resonator (Q ≈ 10\(^4\)-10\(^5\)) with which we find superradiant stimulated microwave emission. In summary, SiC with optimized spin defect density and thus spin relaxation rates is well on its way of becoming a suitable maser gain material with wide-ranging applications.}, language = {en} } @article{SperlichAuthDyakonov2022, author = {Sperlich, Andreas and Auth, Michael and Dyakonov, Vladimir}, title = {Charge transfer in ternary solar cells employing two fullerene derivatives: where do electrons go?}, series = {Israel Journal of Chemistry}, volume = {62}, journal = {Israel Journal of Chemistry}, number = {7-8}, doi = {10.1002/ijch.202100064}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-257506}, year = {2022}, abstract = {Earlier reports demonstrated that ternary organic solar cells (OSC) made of donor polymers (D) blended with different mixtures of fullerene acceptors (A : A) performed very similarly. This finding is surprising, as the corresponding fullerene LUMO levels are slightly different, which might result in decisive differences in the charge transfer step. We investigate ternary OSC (D : A : A) made of the donor polymer P3HT with stoichiometric mixtures of different fullerene derivatives, PC\(_{60}\)BM : PC\(_{70}\)BM and PC\(_{70}\)BM : IC\(_{60}\)BA, respectively. Using quantitative electron paramagnetic resonance (EPR) we can distinguish between positive and negative polarons, localized on the specific molecules. We found that after the initial charge transfer step, the electrons are re-distributed over two nearby acceptors in agreement with their stoichiometry and their relative LUMO energy difference. Remarkably, the measured ΔLUMO differences in fullerene mixtures are reduced by an order of magnitude compared to that of the pristine materials, i. e., below 1 meV for PC\(_{60}\)BM : PC\(_{70}\)BM and (20±5) meV for PC\(_{70}\)BM : IC\(_{60}\)BA. Furthermore, we found that this reduced ΔLUMO explains the shift in open circuit voltage for D : A : A organic solar cells. We attribute these findings to hybridization, leading to an effective fullerene LUMO. Consequently, multi-acceptor blends are indeed a viable option for photodetectors and solar cells, as they combine the best electron acceptor and light absorbing properties.}, language = {en} } @phdthesis{Bathon2021, author = {Bathon, Thomas}, title = {Gezielte Manipulation Topologischer Isolatoren}, doi = {10.25972/OPUS-23920}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-239204}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {Neue physikalische Erkenntnisse vervollst{\"a}ndigen die Sicht auf die Welt und erschließen gleichzeitig Wege f{\"u}r Folgeexperimente und technische Anwendungen. Das letzte Jahrzehnt der Festk{\"o}rperforschung war vom zunehmenden Fokus der theoretischen und experimentellen Erkundung topologischer Materialien gepr{\"a}gt. Eine fundamentale Eigenschaft ist ihre Resistenz gegen{\"u}ber solchen St{\"o}rungen, welche spezielle physikalische Symmetrien nicht verletzen. Insbesondere die Topologischen Isolatoren - Halbleiter mit isolierenden Volumen- sowie gleichzeitig leitenden und spinpolarisierten Oberfl{\"a}chenzust{\"a}nden - sind vielversprechende Kandidaten zur Realisierung breitgef{\"a}cherter spintronischer Einsatzgebiete. Bis zur Verwirklichung von Quantencomputern und anderer, heute noch exotisch anmutender Konzepte bedarf es allerdings ein umfassenderes Verst{\"a}ndnis der grundlegenden, physikalischen Zusammenh{\"a}nge. Diese kommen vor allem an Grenzfl{\"a}chen zum Tragen, weshalb oberfl{\"a}chensensitive Methoden bei der Entdeckung der Topologischen Isolatoren eine wichtige Rolle spielten. Im Rahmen dieser Arbeit werden daher strukturelle, elektronische und magnetische Eigenschaften Topologischer Isolatoren mittels Tieftemperatur-Rastertunnelmikroskopie und -spektroskopie sowie begleitenden Methoden untersucht. Die Ver{\"a}nderung der Element-Ausgangskonzentration w{\"a}hrend dem Wachstum des prototypischen Topologischen Isolators Bi2Te3 f{\"u}hrt zur Realisierung eines topologischen p-n {\"U}bergangs innerhalb des Kristalls. Bei einem spezifischen Verh{\"a}ltnis von Bi zu Te in der Schmelze kommt es aufgrund unterschiedlicher Erstarrungstemperaturen der Komponenten zu einer Ansammlung von Bi- und Te-reichen Gegenden an den gegen{\"u}berliegenden Enden des Kristalls. In diesen bildet sich infolge des jeweiligen Element{\"u}berschusses durch Kristallersetzungen und -fehlstellen eine Dotierung des Materials aus. Daraus resultiert die Existenz eines {\"U}bergangsbereiches, welcher durch Transportmessungen verifiziert werden kann. Mit der r{\"a}umlich aufl{\"o}senden Rastertunnelmikroskopie wird diese Gegend lokalisiert und strukturell sowie elektronisch untersucht. Innerhalb des {\"U}bergangsbereiches treten charakteristische Kristalldefekte beider Arten auf - eine Defektunterdr{\"u}ckung bleibt folglich aus. Dennoch ist dort der Beitrag der Defekte zum Stromtransport aufgrund ihres gegens{\"a}tzlichen Dotiercharakters vernachl{\"a}ssigbar, sodass der topologische Oberfl{\"a}chenzustand die maßgeblichen physikalischen Eigenschaften bestimmt. Dar{\"u}ber hinaus tritt der {\"U}bergangsbereich in energetischen und r{\"a}umlichen Gr{\"o}ßenordnungen auf, die Anwendungen bei Raumtemperatur denkbar machen. Neben der Ver{\"a}nderung Topologischer Isolatoren durch den gezielten Einsatz intrinsischer Kristalldefekte bieten magnetische St{\"o}rungen die M{\"o}glichkeit zur Pr{\"u}fung des topologischen Oberfl{\"a}chenzustandes auf dessen Widerstandsf{\"a}higkeit sowie der gegenseitigen Wechselwirkungen. Die Zeitumkehrinvarianz ist urs{\"a}chlich f{\"u}r den topologischen Schutz des Oberfl{\"a}chenzustandes, weshalb magnetische Oberfl{\"a}chen- und Volumendotierung diese Symmetrie brechen und zu neuartigem Verhalten f{\"u}hren kann. Die Oberfl{\"a}chendotierung Topologischer Isolatoren kann zu einer starken Bandverbiegung und einer energetischen Verschiebung des Fermi-Niveaus f{\"u}hren. Bei einer wohldosierten Menge der Adatome auf p-dotiertem Bi2Te3 kommt die Fermi-Energie innerhalb der Volumenzustands-Bandl{\"u}cke zum Liegen. Folglich wird bei Energien rund um das Fermi-Niveau lediglich der topologische Oberfl{\"a}chenzustand bev{\"o}lkert, welcher eine Wechselwirkung zwischen den Adatomen vermitteln kann. F{\"u}r Mn-Adatome kann R{\"u}ckstreuung beobachtet werden, die aufgrund der Zeitumkehrinvarianz in undotierten Topologischen Isolatoren verboten ist. Die {\"u}berraschenderweise starken und fokussierten Streuintensit{\"a}ten {\"u}ber mesoskopische Distanzen hinweg resultieren aus der ferromagnetischen Kopplung nahegelegener Adsorbate, was durch theoretische Berechnungen und R{\"o}ntgendichroismus-Untersuchungen best{\"a}tigt wird. Gleichwohl wird f{\"u}r die Proben ein superparamagnetisches Verhalten beobachtet. Im Gegensatz dazu f{\"u}hrt die ausreichende Volumendotierung von Sb2Te3 mit V-Atomen zu einem weitreichend ferromagnetischen Verhalten. Erstaunlicherweise kann trotz der weitl{\"a}ufig verbreiteten Theorie Zeitumkehrinvarianz-gebrochener Dirac-Zust{\"a}nde und der experimentellen Entdeckung des Anormalen Quanten-Hall-Effektes in {\"a}hnlichen Probensystemen keinerlei Anzeichen einer spektroskopischen Bandl{\"u}cke beobachtet werden. Dies ist eine direkte Auswirkung der dualen Natur der magnetischen Adatome: W{\"a}hrend sie einerseits eine magnetisch induzierte Bandl{\"u}cke {\"o}ffnen, besetzen sie diese durch St{\"o}rstellenresonanzen wieder. Ihr stark lokaler Charakter kann durch die Aufnahme ihrer r{\"a}umlichen Verteilung aufgezeichnet werden und f{\"u}hrt zu einer Mobilit{\"a}ts-Bandl{\"u}cke, deren Indizien durch vergleichende Untersuchungen an undotiertem und dotiertem Sb2Te3 best{\"a}tigt werden.}, subject = {Rastertunnelmikroskopie}, language = {de} } @phdthesis{Youssef2022, author = {Youssef, Almoatazbellah}, title = {Fabrication of Micro-Engineered Scaffolds for Biomedical Application}, doi = {10.25972/OPUS-23545}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-235457}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Thermoplastic polymers have a history of decades of safe and effective use in the clinic as implantable medical devices. In recent years additive manufacturing (AM) saw increased clinical interest for the fabrication of customizable and implantable medical devices and training models using the patients' own radiological data. However, approval from the various regulatory bodies remains a significant hurdle. A possible solution is to fabricate the AM scaffolds using materials and techniques with a clinical safety record, e.g. melt processing of polymers. Melt Electrowriting (MEW) is a novel, high resolution AM technique which uses thermoplastic polymers. MEW produces scaffolds with microscale fibers and precise fiber placement, allowing the control of the scaffold microarchitecture. Additionally, MEW can process medical-grade thermoplastic polymers, without the use of solvents paving the way for the production of medical devices for clinical applications. This pathway is investigated in this thesis, where the layout is designed to resemble the journey of a medical device produced via MEW from conception to early in vivo experiments. To do so, first, a brief history of the development of medical implants and the regenerative capability of the human body is given in Chapter 1. In Chapter 2, a review of the use of thermoplastic polymers in medicine, with a focus on poly(ε-caprolactone) (PCL), is illustrated, as this is the polymer used in the rest of the thesis. This review is followed by a comparison of the state of the art, regarding in vivo and clinical experiments, of three polymer melt AM technologies: melt-extrusion, selective laser sintering and MEW. The first two techniques already saw successful translation to the bedside, producing patient-specific, regulatory-approved AM implants. To follow in the footsteps of these two technologies, the MEW device parameters need to be optimized. The MEW process parameters and their interplay are further discussed in Chapter 3 focusing on the importance of a steady mass flow rate of the polymer during printing. MEW reaches a balance between polymer flow, the stabilizing electric field and moving collector to produce reproducible, high-resolution scaffolds. An imbalance creates phenomena like fiber pulsing or arcing which result in defective scaffolds and potential printer damage. Chapter 4 shows the use of X-ray microtomography (µCT) as a non-destructive method to characterize the pore-related features: total porosity and the pore size distribution. MEW scaffolds are three-dimensional (3D) constructs but have long been treated in the literature as two-dimensional (2D) ones and characterized mainly by microscopy, including stereo- and scanning electron microscopy, where pore size was simply reported as the distance between the fibers in a single layer. These methods, together with the trend of producing scaffolds with symmetrical pores in the 0/90° and 0/60/120° laydown patterns, disregarded the lateral connections between pores and the potential of MEW to be used for more complex 3D structures, mimicking the extracellular matrix. Here we characterized scaffolds in the aforementioned symmetrical laydown patterns, along with the more complex 0/45/90/135° and 0/30/60/90/120/150° ones. A 2D pore size estimation was done first using stereomicroscopy, followed by and compared to µCT scanning. The scaffolds with symmetrical laydown patterns resulted in the predominance of one pore size, while those with more complex patterns had a broader distribution, which could be better shown by µCT scans. Moreover, in the symmetrical scaffolds, the size of 3D pores was not able to reach the value of the fiber spacing due to a flattening effect of the scaffold, where the thickness of the scaffold was less than the fiber spacing, further restricting the pore size distribution in such scaffolds. This method could be used for quality assurance of fabricated scaffolds prior to use in in vitro or in vivo experiments and would be important for a clinical translation. Chapter 5 illustrates a proof of principle subcutaneous implantation in vivo experiment. MEW scaffolds were already featured in small animal in vivo experiments, but to date, no analysis of the foreign body reaction (FBR) to such implants was performed. FBR is an immune reaction to implanted foreign materials, including medical devices, aimed at protecting the host from potential adverse effects and can interfere with the function of some medical implants. Medical-grade PCL was used to melt electrowrite scaffolds with 50 and 60 µm fiber spacing for the 0/90° and 0/60/120° laydown patterns, respectively. These implants were implanted subcutaneously in immunocompetent, outbred mice, with appropriate controls, and explanted after 2, 4, 7 and 14 days. A thorough characterization of the scaffolds before implantation was done, followed by a full histopathological analysis of the FBR to the implants after excision. The scaffolds, irrespective of their pore geometry, induced an extensive FBR in the form of accumulation of foreign body giant cells around the fiber walls, in a manner that almost occluded available pore spaces with little to no neovascularization. This reaction was not induced by the material itself, as the same reaction failed to develop in the PCL solid film controls. A discussion of the results was given with special regard to the literature available on flat surgical meshes, as well as other hydrogel-based porous scaffolds with similar pore sizes. Finally, a general summary of the thesis in Chapter 6 recapitulates the most important points with a focus on future directions for MEW.}, language = {en} } @phdthesis{Martin2021, author = {Martin, Konstantin}, title = {Current-induced Magnetization Switching by a generated Spin-Orbit Torque in the 3D Topological Insulator Material HgTe}, doi = {10.25972/OPUS-24049}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-240490}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {Magnetic random access memory (MRAM) technology aims to replace dynamic RAM (DRAM) due to its significantly lower power consumption and non-volatility [Dong08]. During the last couple of years the commercial focus was set on spin-transfer torque MRAM (STT-MRAM) systems, where a current is pushed through a ferromagnetic (FM) free layer and a reference layer which are separated by an insulator. The free layer can be set to parallel or anti-parallel depending on the current direction [Kim11]. Unfortunately these currents have to be quite high which could lead to damages of the tunnel barrier of the magnetic tunnel junction resulting in higher power consumption as well as reliability issues. At this point a new effect, where the current is passed below the ferromagnetic layer stack, can be exploited to change the direction of the free layer magnetization. The effect is known as spin-orbit torque (SOT) and describes the transfer of angular momentum onto an adjacent magnetization either by the spin Hall effect (SHE) or inverse spin galvanic effect (iSGE) [Manchon19]. The latter describes a spin accumulation due to a current. This is similar to the process of spin accumulation in TIs, where a current corresponds to an effective spin due to spin-momentum locking [Qi11]. Thus TIs exhibit a high current-to-spin conversion rate, which makes them a promising material system for SOT experiments. Among all TIs it is HgTe, which can be reliably grown as an insulator. This thesis covers the development of a working device for SOT measurements (SOT-device) in a CdTe/CdHgTe/HgTe/CdHgTe heterostructure. It involves the development of a tunnel barrier (ZrOx) as well as the investigation of the behavior of a ferromagnetic layer stack on top of etched HgTe. The main result of this work is the successful construction and evaluation of a working SOT-device, which exhibits the up to date most efficient switching of in-plane magnetized ferromagnetic layer stacks. In order to avoid hybridization between HgTe and the adjacent ferromagnetic atoms, which would cause a breakdown of the topological surface state, it is necessary to implement a thin tunnel barrier in between the TI and free layer [Zhang16]. Aside from hybridization a tunnel barrier avoids shunting of the current, that is pushed on the surface of the HgTe/CdHgTe interface. Thus a bigger part of the current can be used for spin accumulation and, at the same time, the resistance measurement of the ferromagnetic layer stack is not perturbed. In chapter 3 the focus is set on investigating the tunneling characteristics of ZrOx on top of dry etched HgTe. Thin barriers are used as the interaction of the current generated spin and the adjacent magnetization decreases with distance. On the other hand too small insulator thicknesses lead to leakage currents which disturb heavily the measurement of the resistance of the ferromagnetic layer stack. Thus an optimum thickness of 10 ALD cycles (\(d\approx 1.6\rm\, nm\)) is determined which yields a resistance area product of \(R\cdot A \approx 3\rm\, k\Omega\mu m^{2}\). This corresponds to a tunneling resistance of \(R_{T}\approx 20\rm\, k\Omega\) over a structure surface of \(A_{T} = 0.12\rm\, \mu m^2\). Multiple samples with different thicknesses have been produced. All samples have been examined on their tunneling behavior. The resistance area product as a function of thickness shows a linear behavior on a logarithmic scale. Furthermore all working samples show non-linear I-V curves as well as parabolic dI/dV-curves. Additionally the tunneling resistance \(R_{T}\) increases with decreasing temperature. All above mentioned properties are typical for tunnel barriers which do not include pinholes [Jonsson00]. The last part of chapter 3 deals with thermal properties of HgTe. By measuring the second harmonic of a biasing AC current in the channel below the tunnel barrier it is attempted to extract the diffusion thermopower of the heated electrons. Unfortunately the measured signal showed a far superior contribution of the first harmonic. According to electric circuit simulations a small asymmetry in the barrier (penetration and leaving point of electrons) could be responsible for this behavior. A ferromagnetic layer stack, consisting of PY/Cu/CoFe, serves as a sensor for magnetization changes due to external fields and current induced spin accumulations. The layer stack exhibits a giant magnetoresistance (GMR) which has been measured by a resistance bridge. The biggest peculiarity in depositing a GMR stack on top of HgTe is that its easy axis forms along only one of the crystal axes (\((110)\) or \((1\overline{1}0)\)). The reason for this anisotropy is still unclear. Sources such as an influence of the terminating material, miscut, furrows during IBE or sputter ripples have been ruled out. It can be speculated that the surface states due to HgTe might have an influence on the development of this easy axis but this would need further investigation. A consequence of this unexpected anisotropy is that every CdTe/CdHgTe/HgTe/CdHgTe wafer has first to be characterized in SQUID in order to find the easy axis. A ferromagnetic resonance (FMR) measurement confirmed this observation. The shape of the ferromagnetic layer stack is chosen to be an ellipse in order to support the easy axis direction by shape anisotropy. Over 8 million ellipses are used to generate a SQUID signal of \(m > 10^{-5}\rm\, emu\). This is sufficient to extract the main characteristics of an average nano pillar under the influence of an external magnetic field. As in the case of bigger structures the ellipse shaped structure shows a step-like behavior. A measured minor loop confirms the existence of the irreversible anti-parallel stable magnetic state. Furthermore this state persists for both directions at \(m=0\) resulting in an anti-ferromagnetic coupling between Py and CoFe. The geometry of the SOT-device is chosen in such a way that the current induced spin aligns either parallel or anti-parallel to the effective magnetic field \(\vec{B}_{eff}=\vec{B}_{ext}+\vec{B}_{aniso}+\vec{B}_{shape}\), which acts on the pillar. Due to interaction of the spin with the adjacent magnetization of Py the magnetization direction gets changed by a torque \(\vec{T}\). In general this torque can be decomposed into two components a field-like torque \(\vec{\tau}_{FL}\) and a damping-like torque \(\vec{\tau}_{DL}\) [Manchon19]. In the case of TIs \(\vec{T}\) is additionally depending on the z-component of \(\vec{m}\) [Ndiaye17]. In our case the magnetization is lying in the sample plane (\(m_{z}=0\)) which results in \(\vec{\tau}_{DL}=0\). Thus, in the case of \(\vec{S}\parallel\left(\vec{\hat{z}}\times\vec{j}\right)\) and \(\vec{j}\parallel\vec{\hat{y}}\), the only spin dependent effective magnetic field is \(\vec{B}_{FL}=\tau_{FL}\cdot\vec{\hat{x}}\) which is lying parallel or anti-parallel to \(\vec{B}_{eff}\). The evaluation of \(\vec{B}_{FL}\) can therefore be done in the following manner. First a high \(B_{ext}\) has to be set along the easy axis of the pillar. Then \(B_{ext}\) has to be reduced just a few \(\rm\, Oe\) before the switching occurs at the magnetic field \(B_{ext,0}\). At the magnetic field \(\Delta B = B_{ext}-B_{ext,0}\approx 0.5\rm\, Oe\) the lower resistive state should be stable over a longer time range (\(10-30\rm\, min\)) in order to exclude switching due to fluctuations. Now a positive or negative current can be pushed through the channel below the pillar. For one of the two current directions the magnetization of Py switches. It is therefore not a thermal effect that drives the change of \(\vec{m}\). Current densities that are able to switch \(\vec{m}\) at small \(\Delta B\neq 0\) lie in the range of \(j\approx 10^{4}\rm\, A/cm^{2}\). In all experiments the switching efficiency \(\Delta B/j\) decreases with rising \(j\). Furthermore the efficiency as a function of \(j\) depends on the temperature as \(\Delta B/j\) values tend to be up to 20 times higher at \(T=1.8\rm\, K\) and \(j\approx 0\) than at \(T=4.2\rm\, K\). This temperature dependence suggests that switching occurs not due to Oersted fields. Furthermore the Biot-Savart fields had been calculated for four different models: an infinite long rectangular wire, two infinite planes, a full volume and two thin volume planes. Every model shows an efficiency, which is at least three times lower than the observation. The highest efficiencies in our samples show up to 10 times higher values than in heavy-metal/ferromagnets heterostructures. In contrast to measurement procedures of most other groups our method leads to direct determination of SOT parameters like the effective magnetic field \(\vec{B}_{FL}\). Other groups make use of spin-transfer FMR (ST-FMR) where they AC bias their structure and extract SOT parameters (like \(\tau_{FL}\) and \(\tau_{DL}\)) from second harmonics by fitting theoretical models. Material systems consisting of TIs and magnetic insulators (MIs) on the other hand show 10 times higher efficiencies [Khang18,Li19]. In those cases the magnetization points out of the sample plane which is conceptually different from in-plane magnetic anisotropy geometries like in our case. The greatest benefit in-plane magnetic anisotropy systems is its easy realisation [Bhatti17]. Here only an elliptical shape has to be lithographically implemented instead of conducting research on the appropriate combination of material systems that result in perpendicular magnetic anisotropies [Apalkov16]. Despite the fact that in our case only \(\vec{\tau}_{FL}\) acts as the driving force for changing \(m\) our device still exhibits the up to date highest efficiencies in the class of in-plane magnetized anisotropies of all material classes ever recorded.}, language = {en} } @phdthesis{Scheuermann2021, author = {Scheuermann, Julian}, title = {Interbandkaskadenlaser f{\"u}r Anwendungen in der Absorptionsspektroskopie}, doi = {10.25972/OPUS-25179}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-251797}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {Das Ziel dieser Arbeit war die Entwicklung und Weiterentwicklung von Laserlichtquellen basierend auf der Interbandkaskadentechnologie in einem Wellenl{\"a}ngenbereich von ca. 3 bis 6 µm. Der Fokus lag dabei auf der Entwicklung von Kantenemitter-Halbleiterlasern, welche bei verschiedensten Emissionswellenl{\"a}ngen erfolgreich hergestellt werden konnten. Dabei wurde auf jeweilige Herausforderungen eingegangen, welche entweder durch die Herstellung selbst oder der anwendungstechnischen Zielsetzung bedingt war. Im Rahmen dieser Arbeit wurden verschiedene, spektral einzelmodige Halbleiterlaser im angesprochenen Wellenl{\"a}ngenbereich entwickelt und hergestellt. Basierend auf dem jeweiligen Epitaxiematerial und der angestrebten Emissionswellenl{\"a}nge wurden Simulationen der optischen Lasermode durchgef{\"u}hrt und die grundlegenden f{\"u}r die Herstellung notwendigen Parameter bestimmt und experimentell umgesetzt. Des Weiteren wurden die verwendeten Verfahren f{\"u}r den jeweiligen Herstellungsprozess angepasst und optimiert. Das umfasst die in den ersten Kapiteln beschriebenen Schritte wie optische Lithografie, Elektronenstrahllithografie, reaktives Trocken{\"a}tzen und verschiedene Arten der Materialdeposition. Mit einer Emissionswellenl{\"a}nge von 2,8 µm wurde beispielsweise der bislang kurzwelligste bei Raumtemperatur im Dauerstrichbetrieb betriebene einzelmodige Interbandkaskadenlaser hergestellt. Dessen Leistungsmerkmale sind mit Diodenlasern im entsprechenden Emissionsbereich vergleichbar. Somit erg{\"a}nzt die Interbandkaskadentechnologie bestehende Technologien nahtlos und es ist eine l{\"u}ckenlose Wellenl{\"a}ngenabdeckung bis in den mittleren Infrarotbereich m{\"o}glich. Je nach Herstellungsprozess wurde außerdem auf die verteilte R{\"u}ckkopplung eingegangen und die Leistungsf{\"a}higkeit des verwendeten Metallgitterkonzeptes anhand von Messungen an spektral einzelmodigen Bauteile aufgezeigt. Es wurden aber auch die je nach Zielsetzung unterschiedlichen Herausforderungen aufgezeigt und diskutiert. F{\"u}r eine Anwendung wurden spezielle Laserchips mit zwei einzelmodigen Emissionswellenl{\"a}ngen bei 3928 nm und 4009 nm entwickelt. Die beiden Wellenl{\"a}ngen sind f{\"u}r die Detektion von Schwefeldioxid und Schwefelwasserstoff geeignet, welche zur {\"U}berwachung und Optimierung der Schwefelgewinnung durch das Claus-Verfahren notwendig sind. Bei der Umsetzung wurden auf einzelnen Chips zwei Laseremitter in einem Abstand von 70 µm platziert und mit je einem Metallgitter versehen. Das verwendete Epitaxiematerial war so konzipiert, dass es optimal f{\"u}r beide Zielwellenl{\"a}ngen verwendet werden kann. Die geforderten Eigenschaften wurden erf{\"u}llt und die Bauteile konnten erfolgreich hergestellt werden. Die Emissionseigenschaften und das spektrale Verhalten wurde bei beiden Zielwellenl{\"a}ngen bestimmt. Einzeln betrachtet erf{\"u}llen beide Emitter die notwendigen Eigenschaften um f{\"u}r spektroskopische Anwendungen eingesetzt werden zu k{\"o}nnen. Erg{\"a}nzend wurde zum einen das Abstimmverhalten der Emissionswellenl{\"a}nge in Abh{\"a}ngigkeit der Modulationsfrequenz des Betriebsstromes untersucht und zus{\"a}tzlich die thermische Abh{\"a}ngigkeit der Betriebsparameter beider Kan{\"a}le zueinander bestimmt. Diese Abh{\"a}ngigkeit ist f{\"u}r eine simultane Messung mit beiden Kan{\"a}len notwendig. Das Konzept mit mehreren Stegwellenleitern pro Laserchip wurde in einem weiteren Fall noch st{\"a}rker ausgearbeitet. Denn je nach Komplexit{\"a}t eines Gasgemisches sind zur Bestimmung der einzelnen Komponenten mehr Messpunkte bzw. Wellenl{\"a}ngen notwendig. Im zweiten Fall ist die Analyse der Kohlenwasserstoffe Methan, Ethan, Propan, Butan, Iso-Butan, Pentan und Iso-Pentan von Interesse, welche als Hauptbestandteile von Erdgas z.B. in Erdgasaufbereitungsanlagen oder zur Bestimmung des Heizwertes analysiert werden m{\"u}ssen. Die genannten Kohlenwasserstoffe zeigen ein starkes Absorptionsverhalten im Wellenl{\"a}ngenbereich von 3,3 bis 3,5 µm. Auf dem entsprechend angepassten Interbandkaskadenmaterial wurden Bauteile mit neun Wellenleitern pro Laserchip hergestellt. Mithilfe der neun einzelmodigen Emissionskan{\"a}le konnte ein Bereich von bis zu 190 nm (21 meV, 167 cm-1) adressiert werden. Außerdem wurde der sich mit zunehmender Wellenl{\"a}nge {\"a}ndernde Schichtaufbau und dessen Einfluss auf die Bauteileigenschaften diskutiert. Die Leistungsdaten der langwelligsten Epitaxie waren im Vergleich deutlich schw{\"a}cher. Um diesen Nachteil zu kompensieren, wurde eine spezielle Wellenleitergeometrie mit doppeltem Steg genutzt. Die Eigenschaften des Konzeptes wurden zuerst mittels Simulation untersucht und ein entsprechendes Herstellungsverfahren entwickelt. Mit der Simulation als Grundlage wurden die verschiedenen Prozessparameter {\"u}ber mehrere Prozessl{\"a}ufe iterativ optimiert und somit die Performance der Laser verbessert. Auch mit diesem Verfahren konnte ausreichende Kopplung an das Metallgitter erzielt werden. Abschließend wurden mit diesem Herstellungsverfahren einzelmodige Laser im Wellenl{\"a}ngenbereich von 5,9 bis {\"u}ber 6 Mikrometern realisiert. Diese Laser emittierten im Dauerstrichbetrieb bei einer maximalen Betriebstemperatur von -2 °C. Insgesamt wurde anhand der im Rahmen dieser Arbeit entwickelten Bauteilen und de ren Charakterisierung gezeigt, dass diese die Anforderungen von TLAS Anwendungen erf{\"u}llen. Jedoch konnte nur auf einen Teil der M{\"o}glichkeiten eingegangen werden, den die Interbandkaskadentechnologie bietet, denn die angesprochenen Einsatzgebiete stellen nur einzelne grundlegende M{\"o}glichkeiten dieser Technologie mit Schwerpunkt auf laserbasierte Lichtquellen dar. Zusammenfassend kann allerdings gesagt werden, dass sich die Interbandkaskadentechnologie etabliert hat. Gerade durch die gezeigten Leistungsdaten bei den Wellenl{\"a}ngen um 2,9 µm, 3,4 µm und 4,0 µm im Dauerstrichbetrieb bei Raumtemperatur wird ersichtlich, dass im Bereich der Sensorik die ICL Technologie in Bezug auf niedriger Strom- bzw. Leistungsaufnahme quasi konkurrenzlos ist. Sicherlich werden die Anwendungsgebiete in Zukunft noch vielf{\"a}ltiger. Denn es sind auf jeden Fall weitere Fortschritte in Richtung h{\"o}herer Emissionswellenl{\"a}ngen, deutlich h{\"o}herer Betriebstemperaturen, verbreiterte Emissionsbereiche oder g{\"a}nzlich andere Bauteil Konzepte wie z.B. f{\"u}r Frequenzk{\"a}mme bzw. Terahertz Anwendungen zu erwarten. Diese Entwicklung betrifft nicht nur den Einsatz als Lichtquelle, denn auch Interbandkaskadendetektoren bzw. Solarzellen wurden schon realisiert und werden weiterentwickelt.}, subject = {Halbleiterlaser}, language = {de} } @phdthesis{Kudriashova2019, author = {Kudriashova, Liudmila}, title = {Photoluminescence Reveals Charge Carrier Recombination in Organic and Hybrid Semiconductors}, doi = {10.25972/OPUS-19343}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-193437}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {In this work, we elucidated recombination kinetics in organic and hybrid semiconductors by steady-state and time-resolved PL spectroscopy. Using these simple and very flexible experimental techniques, we probed the infrared emission from recombining free charge carriers in metal-halide perovskites, as well as the deep blue luminescence from intramolecular charge-transfer states in novel OLED emitters. We showed that similar state diagrams and kinetic models accurately describe the dynamics of excited species in these very different material systems. In Chapters 4 and 5, we focused on lead iodide perovskites (MAPI and FAPI), whose comparatively developed deposition techniques suited the systematic material research. In MAPI, we harnessed the anomalous dependence of transient PL on the laser repetition rate in order to investigate the role of interfaces with the commonly used charge-selective layers: PC60BM, spiro-MeOTAD, and P3HT. The film was deposited on a large precut substrate and separated into several parts, which were then covered with the charge-selective layers. Thereby, the same bulk perovskite structure was maintained for all samples. Consequently, we were able to isolate interface-affected and bulk carrier recombination. The first one dominated the fast component of PL decay up to 300 ns, whereas the last was assigned to the remaining slow component. The laser repetition rate significantly prolonged PL decay in MAPI with additional interfaces while shortening the charge carrier lifetime in the pristine film. We qualitatively explained this effect by a kinetic model that included radiative electron-hole recombination and nonradiative trap-assisted recombination. All in all, we showed that the apparent PL lifetime in MAPI is to large extend defined by the laser repetition rate and by the adjacent interfaces. Further, we studied photon recycling in MAPI and FAPI. We monitored how the microscopic PL transforms while propagating through the thin perovskite film. The emission was recorded within 5orders of magnitude in intensity up to 70μm away from the excitation spot. The Beer-Lambert law previously failed to describe the complex interplay of the intrinsic PL spectrum and the additional red-shifted peak. Therefore, we developed a general numerical model that accounts for self-absorption and diffusion of the secondary charge carriers. A simulation based on this model showed excellent agreement with the experimental spatially resolved PL maps. The proposed model can be applied to any perovskite film, because it uses easily measurable intrinsic PL spectrum and macroscopic absorption coefficient as seeding parameters. In Chapter 6, we conducted an extensive photophysical study of a novel compact deep blue OLED emitter, SBABz4, containing spiro-biacridine and benzonitrile units. We also considered its single-donor monomer counterpart, DMABz4, in order to highlight the structure-property relationships. Both compounds exhibited thermally activated delayed fluorescence (TADF), which was independently proven by oxygen quenching and temperature-dependent transient PL measurements. The spiro-linkage in the double-donor core of SBABz4 rendered its luminescence pure blue compared to the blue-green emission from the single-donor DMABz4. Thus, the core-donor provided desirable color tuning in the deep blue region, as opposed to the common TADF molecular design with core-acceptor. Using PL lifetimes and efficiencies, we predicted EQEmax = 7.1\% for SBABz4-based OLED, whereas a real test device showed EQEmax = 6.8\%. Transient PL was recorded from the solutions and solid films in the unprecedentedly broad dynamic range covering up to 6orders of magnitude in time and 8orders of magnitude in intensity. The stretched exponent was shown to fit the transient PL in the films very well, whereas PL decay in dilute solution was found purely exponential. When the emitter was embedded in the host matrix that prevented aggregation, its TADF properties were superior in comparison with the pure SBABz4 film. Finally, using temperature-dependent transient PL data, we calculated the TADF activation energy of 70 meV. To sum up, this Thesis contributes to the two fascinating topics of the last decade's material research: perovskite absorbers for photovoltaics and TADF emitters for OLEDs. We were lucky to work with the emerging systems and tailor for them new models out of the well-known physical concepts. This was both exciting and challenging. In the end, science of novel materials is always a mess. We hope that we brought there a bit of clarity and light.}, subject = {Time-resolved photoluminescence}, language = {en} } @phdthesis{Raghuraman2020, author = {Raghuraman, Sairamesh}, title = {New RF coil arrays for Static and Dynamic Musculoskeletal Magnetic Resonance Imaging}, doi = {10.25972/OPUS-20416}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-204165}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2020}, abstract = {Magnetic Resonance Imaging at field strengths up to 3 T, has become a default diagnostic modality for a variety of disorders and injuries, due to multiple reasons ranging from its non-invasive nature to the possibility of obtaining high resolution images of internal organs and soft tissues. Despite tremendous advances, MR imaging of certain anatomical regions and applications present specific challenges to be overcome. One such application is MR Musculo-Skeletal Imaging. This work addresses a few difficult areas within MSK imaging from the hardware perspective, with coil solutions for dynamic imaging of knee and high field imaging of hand. Starting with a brief introduction to MR physics, different types of RF coils are introduced in chapter 1, followed by sections on design of birdcage coils, phased arrays and their characterization in chapter 2. Measurements, calculations and simulations, done during the course of this work, have been added to this chapter to give a quantitative feel of the concepts explained. Chapter 3 deals with the construction of a phased array receiver for dynamic imaging of knee of a large animal model, i.e. minipig, at 1.5 T. Starting with details on the various aspects of an application that need to be considered when an MR RF array is designed, the chapter details the complex geometry of the region of interest in a minipig and reasons that necessitate a high density array. The sizes of the individual elements that constitute the array have been arrived at by studying the ratio of unloaded to loaded Q factors and choosing a size that provides the best ratio but still maintains a uniform SNR throughout the movement of the knee. To have a minimum weight and to allow mechanical movement of the knee, the Preamplifiers were located in a separate box. A movement device was constructed to achieve adjustable periodic movement of the knee of the anesthetized animal. The constructed array has been characterized for its SNR and compared with an existing product coil to show the improvement. The movement device was also characterized for its reproducibility. High resolution static images with anatomical details marked have been presented. The 1/g maps show the accelerations possible with the array. Snapshots of obtained dynamic images trace the cruciate ligaments through a cycle of movement of the animal's knee. The hardware combination of a high density phased array and a movement device designed for a minipig's knee was used as a 'reference' and extended in chapter 4 for a human knee. In principle the challenges are similar for dynamic imaging of a human knee with regards to optimization of the elements, the associated electronics and the construction of the movement device. The size of the elements were optimized considering the field penetration / sensitivity required for the internal tissues. They were distributed around the curvature of the knee keeping in mind the acceleration required for dynamic imaging and the direction of the movement. The constructed movement device allows a periodic motion of the lower half of the leg, with the knee placed within the coil, enabling visualization of the tissues inside, while the leg is in motion. Imaging has been performed using dynamic interleaved acquisition sequence where higher effective TR and flip angles are achieved due to a combination of interleaving and segmentation of the sequence. The movement device has been characterized for its reproducibility while the SNR distribution of the constructed RF array has been compared with that of a commercially available standard 8 channel array. The results show the improvement in SNR and acceleration with the constructed geometry. High resolution static images, dynamic snapshots and the 3D segmentation of the obtained images prove the usefulness of the complete package provided in the design, for performing dynamic imaging at a clinically relevant field strength. A simple study is performed in chapter 5 to understand the effects of changes in overlap for coil configurations with different loads and at different frequencies. The noise levels of individual channels and the correlation between them are plotted against subtle changes in overlap, at 64 and 123 MHz. SNR for every overlap setup is also measured and plotted. Results show that achieving critical overlap is crucial to obtain the best possible SNR in those coil setups where the load offered by the sample is low. Chapter 6 of the thesis work deals with coil design for high field imaging of hand and wrists at 7 T, with an aim to achieve ultra high resolution imaging. At this field strength due to the increase in dielectric effects and the resulting decrease in homogeneity, whole body transmit coils are impractical and this has led engineers to design local transmit coils, for specific anatomies. While transmit or transceive arrays are usually preferred, to mitigate SAR effects, the spatial resolution obtained is limited. It is shown that a solution to this, with regards to hand imaging, can be a single volume transmit coil, along with high density receive arrays optimized for different regions of the hand. The use of a phased array for reception provides an increased SNR / penetration under high resolution. A volume transmit coil could pose issues in homogeneity at 7 T, but the specific anatomy of hand and wrist, with comparatively less water content, limits dielectric effects to have homogeneous B_1+ profile over the hand. To this effect, a bandpass birdcage and a 12 channel receive array are designed and characterized. Images of very high spatial resolution (0.16 x 0.16 x 0.16 mm3) with internal tissues marked are presented. In vivo 1/g maps show that an acceleration of up to 3 is possible and the EM simulation results presented show the uniform field along with SAR hotspots in the hand. To reduce the stress created due to the 'superman' position of imaging, provisions in the form of a holder and a hand rest have been designed and presented. Factors that contributed to the stability of the presented design are also listed, which would help future designs of receive arrays at high field strengths. In conclusion, the coils and related hardware presented in this thesis address the following two aspects of MSK imaging: Dynamic imaging of knee and High resolution imaging of hand / wrist. The presented hardware addresses specific challenges and provides solutions. It is hoped that these designs are steps in the direction of improving the existing coils to get a better knowledge and understanding of MSK diseases such as Rheumatoid Arthritis and Osteoarthritis. The hardware can aid our study of ligament reconstruction and development. The high density array and transmit coil design for hand / wrist also demonstrates the benefits of the obtained SNR at 7 T while maintaining SAR within limits. This design is a contribution towards optimizing hardware at high field strength, to make it clinically acceptable and approved by regulatory bodies.}, subject = {MRI}, language = {en} } @misc{Gross2022, type = {Master Thesis}, author = {Groß, Lennart}, title = {Point-spread function engineering for single-molecule localization microscopy in brain slices}, doi = {10.25972/OPUS-28259}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-282596}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Single-molecule localization microscopy (SMLM) is the method of choice to study biological specimens on a nanoscale level. Advantages of SMLM imply its superior specificity due to targeted molecular fluorescence labeling and its enhanced tissue preservation compared to electron microscopy, while reaching similar resolution. To reveal the molecular organization of protein structures in brain tissue, SMLM moves to the forefront: Instead of investigating brain slices with a thickness of a few µm, measurements of intact neuronal assemblies (up to 100 µm in each dimension) are required. As proteins are distributed in the whole brain volume and can move along synapses in all directions, this method is promising in revealing arrangements of neuronal protein markers. However, diffraction-limited imaging still required for the localization of the fluorophores is prevented by sample-induced distortion of emission pattern due to optical aberrations in tissue slices from non-superficial planes. In particular, the sample causes wavefront dephasing, which can be described as a summation of Zernike polynomials. To recover an optimal point spread function (PSF), active shaping can be performed by the use of adaptive optics. The aim of this thesis is to establish a setup using a deformable mirror and a wavefront sensor to actively shape the PSF to correct the wavefront phases in a super-resolution microscope setup. Therefore, fluorescence-labeled proteins expressed in different anatomical regions in brain tissue will be used as experiment specimen. Resolution independent imaging depth in slices reaching tens of micrometers is aimed.}, subject = {Einzelmolek{\"u}lmikroskopie}, language = {en} } @phdthesis{Brodbeck2020, author = {Brodbeck, Sebastian}, title = {Elektrische und magnetische Felder zur Untersuchung und Manipulation von Exziton-Polaritonen}, doi = {10.25972/OPUS-20739}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-207397}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2020}, abstract = {Starke Licht-Materie-Wechselwirkung in Halbleiter-Mikroresonatoren f{\"u}hrt zur Ausbildung von Eigenmoden mit gemischtem Licht-Materie-Charakter, die als Polaritonen bezeichnet werden. Die besonderen Eigenschaften dieser bosonischen Quasiteilchen k{\"o}nnen zur Realisierung neuartiger Bauteile genutzt werden, wie etwa des Polariton-Lasers, der auf stimulierter Streuung beruht anstatt auf stimulierter Emission, durch die Photon-Lasing ausgel{\"o}st wird. Durch den direkten Zugang zu Polariton-Zust{\"a}nden in spektroskopischen Experimenten, sowie durch die M{\"o}glichkeit mit vielf{\"a}ltigen Mitteln nahezu beliebige Potentiallandschaften definieren zu k{\"o}nnen, er{\"o}ffnen sich zahlreiche weitere Anwendungsgebiete, etwa in der Quantensimulation bzw. -emulation. Mittels externer elektrischer und magnetischer Felder k{\"o}nnen Erkenntnisse {\"u}ber Polaritonen gewonnen werden, die in rein optischen Experimenten nicht zug{\"a}nglich sind. Durch die Felder, die nicht mit rein photonischen Moden wechselwirken, kann auf den Materie-Anteil der Hybridmoden zugegriffen werden. Weiterhin k{\"o}nnen die Felder zur in-situ Manipulation der Polariton-Energie genutzt werden, was f{\"u}r die Erzeugung dynamischer Potentiale relevant werden k{\"o}nnte. Der Fokus dieser Arbeit liegt daher auf der Betrachtung verschiedener Ph{\"a}nomene der Licht-Materie-Wechselwirkung unter dem Einfluss {\"a}ußerer Felder. Dazu wurden auf das jeweilige Experiment abgestimmte Strukturen und Bauteile hergestellt und in magneto-optischen oder elektro-optischen Messungen untersucht. Um elektrische Felder entlang der Wachstumsrichtung anlegen zu k{\"o}nnen, d.h. in vertikaler Geometrie, wurden dotierte Resonatoren verwendet, die mit elektrischen Kontakten auf der Probenoberfl{\"a}che und -r{\"u}ckseite versehen wurden. In diesen Bauteilen wurde die Energieverschiebung im elektrischen Feld untersucht, der sogenannte Stark-Effekt. Dieser im linearen Regime bereits mehrfach demonstrierte Effekt wurde systematisch auf den nichtlinearen Bereich des Polariton-Lasings erweitert. Dabei wurde besonderes Augenmerk auf die Probengeometrie und deren Einfluss auf die beobachteten Energieverschiebungen gelegt. Die Untersuchungen von Proben mit planarer, semi-planarer und Mikrot{\"u}rmchen-Geometrie zeigen, dass ein lateraler Einschluss der Ladungstr{\"a}ger, wie er im Mikrot{\"u}rmchen erzielt wird, zu einer Umkehrung der Energieverschiebung f{\"u}hrt. W{\"a}hrend in dieser Geometrie mit zunehmender Feldst{\"a}rke eine Blauverschiebung des unteren Polaritons gemessen wird, die durch Abschirmungseffekte erkl{\"a}rt werden kann, wird in planarer und semi-planarer Geometrie die erwartete Rotverschiebung beobachtet. In beiden F{\"a}llen k{\"o}nnen, je nach Verstimmung, Energieverschiebungen im Bereich von einigen hundert µeV gemessen werden. Die gemessenen Energieverschiebungen zeigen gute {\"U}bereinstimmung mit den Werten, die nach einem Modell gekoppelter Oszillatoren berechnet wurden. Weiterhin werden vergleichbare Energieverschiebungen unter- und oberhalb der Schwelle zum Polariton-Lasing beobachtet, sodass der Polariton-Stark-Effekt als eindeutiges Merkmal erachtet werden kann, anhand dessen optisch angeregte Polariton- und Photon-Laser eindeutig unterschieden werden k{\"o}nnen. Wird das elektrische Feld nicht entlang der Wachstumsrichtung angelegt, sondern senkrecht dazu in der Ebene der Quantenfilme, dann kommt es schon bei geringen Feldst{\"a}rken zur Feldionisation von Elektron-Loch-Paaren. Um diese Feldgeometrie zu realisieren, wurde ein Verfahren entwickelt, bei dem Kontakte direkt auf die durch einen {\"A}tzvorgang teilweise freigelegten Quantenfilme eines undotierten Mikroresonators aufgebracht werden. Durch das Anlegen einer Spannung zwischen den lateralen Kontakten kann die Polariton-Emission unterdr{\"u}ckt werden, wobei sich die Feldabh{\"a}ngigkeit der Polariton-Besetzung durch ein Modell gekoppelter Ratengleichungen reproduzieren l{\"a}sst. Die neuartige Kontaktierung erlaubt es weiterhin den Photostrom in den Quantenfilmen zu untersuchen, der proportional zur Dichte freier Ladungstr{\"a}ger ist. Dadurch l{\"a}sst sich zeigen, dass die zwei Schwellen mit nichtlinearem Anstieg der Emission, die in derartigen Proben h{\"a}ufig beobachtet werden, auf grunds{\"a}tzlich verschiedene Verst{\"a}rkungsmechanismen zur{\"u}ckgehen. An der zweiten Schwelle wird ein Abknicken des leistungsabh{\"a}ngigen Photostroms beobachtet, da dort freie Ladungstr{\"a}ger als Reservoir des Photon-Lasings dienen, deren Dichte an der Schwelle teilweise abgeklemmt wird. Die erste Schwelle hingegen, die dem Polariton-Lasing zugeordnet wird, hat keinen Einfluss auf den linear mit der Anregungsleistung ansteigenden Photostrom, da dort gebundene Elektron-Loch-Paare als Reservoir dienen. Mittels angepasster Ratengleichungsmodelle f{\"u}r Polariton- und Photon-Laser l{\"a}sst sich der ermittelte Verlauf der Ladungstr{\"a}gerdichte {\"u}ber den gesamten Leistungsbereich qualitativ reproduzieren. Abschließend wird durch ein magnetisches Feld der Einfluss der Licht-Materie-Wechselwirkung auf die Elektron-Loch-Bindung im Regime der sehr starken Kopplung beleuchtet. Durch die Messung der diamagnetischen Verschiebung wird der mittlere Elektron-Loch-Abstand von unterem und oberem Polariton f{\"u}r zwei Resonatoren mit unterschiedlich starker Licht-Materie-Wechselwirkung bestimmt. Bei geringer Kopplungsst{\"a}rke werden die Hybridmoden in guter N{\"a}herung als Linearkombinationen der ungekoppelten Licht- und Materie-Moden beschrieben. F{\"u}r den Resonator mit großer Kopplungsst{\"a}rke wird eine starke Asymmetrie zwischen unterem und oberem Polariton beobachtet. Die diamagnetische Verschiebung des oberen Polaritons steigt mit zunehmender Verstimmung auf bis etwa 2,1 meV an, was fast eine Gr{\"o}ßenordnung {\"u}ber der Verschiebung des unteren Polaritons (0,27 meV) bei derselben Verstimmung liegt und die Verschiebung des ungekoppelten Quantenfilms um mehr als den Faktor 2 {\"u}bersteigt. Das bedeutet, dass das untere Polariton durch eine Wellenfunktion beschrieben wird, dessen Materie-Anteil einen verringerten mittleren Elektron-Loch-Abstand aufweist. Im oberen Polariton ist dieser mittlere Radius deutlich gr{\"o}ßer als der eines Elektron-Loch-Paars im ungekoppelten Quantenfilm, was sich durch eine von Photonen vermittelte Wechselwirkung mit angeregten und Kontinuumszust{\"a}nden des Quantenfilms erkl{\"a}ren l{\"a}sst.}, subject = {Drei-F{\"u}nf-Halbleiter}, language = {de} } @phdthesis{Waeldchen2020, author = {W{\"a}ldchen, Felix}, title = {3D Single Molecule Imaging In Whole Cells Enabled By Lattice Light-Sheet Illumination}, doi = {10.25972/OPUS-20711}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-207111}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2020}, abstract = {Single molecule localization microscopy has seen a remarkable growth since its first experimental implementations about a decade ago. Despite its technical challenges, it is already widely used in medicine and biology and is valued as a unique tool to gain molecular information with high specificity. However, common illumination techniques do not allow the use of single molecule sensitive super-resolution microscopy techniques such as direct stochastic optical reconstruction microscopy (dSTORM) for whole cell imaging. In addition, they can potentially alter the quantitative information. In this thesis, I combine dSTORM imaging in three dimensions with lattice lightsheet illumination to gain quantitative molecular information from cells unperturbed by the illumination and cover slip effects. Lattice light-sheet illumination uses optical lattices for beam shaping to restrict the illumination to the detectable volume. I describe the theoretical background needed for both techniques and detail the experimental realization of the system as well as the software that I developed to efficiently evaluate the data. Eventually, I will present key datasets that demonstrate the capabilities of the developed microscope system with and without dSTORM. My main goal here was to use these techniques for imaging the neural cell adhesion molecule (NCAM, also known as CD56) in whole cells. NCAM is a plasma membrane receptor known to play a key role in biological processes such as memory and learning. Combining dSTORM and lattice light-sheet illumination enables the collection of quantitative data of the distribution of molecules across the whole plasma membrane, and shows an accumulation of NCAM at cell-cell interfaces. The low phototoxicity of lattice light-sheet illumination further allows for tracking individual NCAM dimers in living cells, showing a significant dependence of its mobility on the actin skeleton of the cell.}, subject = {Einzelmolek{\"u}lmikroskopie}, language = {en} } @article{DennerPellen2016, author = {Denner, Ansgar and Pellen, Mathieu}, title = {NLO electroweak corrections to off-shell top-antitop production with leptonic decays at the LHC}, series = {Journal of High Energy Phsyics}, volume = {08}, journal = {Journal of High Energy Phsyics}, number = {155}, doi = {10.1007/JHEP08(2016)155}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166415}, year = {2016}, abstract = {For the first time the next-to-leading-order electroweak corrections to the full off-shell production of two top quarks that decay leptonically are presented. This calculation includes all off-shell, non-resonant, and interference effects for the 6-particle phase space. While the electroweak corrections are below one per cent for the integrated cross section, they reach up to 15\% in the high-transverse-momentum region of distributions. To support the results of the complete one-loop calculation, we have in addition evaluated the electroweak corrections in two different pole approximations, one requiring two on-shell top quarks and one featuring two on-shell W bosons. While the former deviates by up to 10\% from the full calculation for certain distributions, the latter provides a very good description for most observables. The increased centre-of-mass energy of the LHC makes the inclusion of electroweak corrections extremely relevant as they are particularly large in the Sudakov regime where new physics is expected to be probed.}, language = {en} } @article{VogelMarkertRueckertetal.2019, author = {Vogel, Patrick and Markert, Jonathan and R{\"u}ckert, Martin A. and Herz, Stefan and Keßler, Benedikt and Dremel, Kilian and Althoff, Daniel and Weber, Matthias and Buzug, Thorsten M. and Bley, Thorsten A. and Kullmann, Walter H. and Hanke, Randolf and Zabler, Simon and Behr, Volker C.}, title = {Magnetic Particle Imaging meets computed tomography: first simultaneous imaging}, series = {Scientific Reports}, volume = {9}, journal = {Scientific Reports}, doi = {10.1038/s41598-019-48960-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-202501}, pages = {12627}, year = {2019}, abstract = {Magnetic Particle Imaging (MPI) is a promising new tomographic modality for fast as well as three-dimensional visualization of magnetic material. For anatomical or structural information an additional imaging modality such as computed tomography (CT) is required. In this paper, the first hybrid MPI-CT scanner for multimodal imaging providing simultaneous data acquisition is presented.}, language = {en} } @article{KiermaschFischerGilEscrigetal.2021, author = {Kiermasch, David and Fischer, Mathias and Gil-Escrig, Lid{\´o}n and Baumann, Andreas and Bolink, Henk J. and Dyakonov, Vladimir and Tvingstedt, Kristofer}, title = {Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment}, series = {Solar RRL}, volume = {5}, journal = {Solar RRL}, number = {11}, doi = {10.1002/solr.202100400}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-258003}, year = {2021}, abstract = {The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V\(_{OC}\)). From a V\(_{OC}\) of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n\(_{id}\)), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley-Read-Hall recombination rates.}, language = {en} } @article{MuellerLuettigMalyetal.2019, author = {Mueller, Stefan and L{\"u}ttig, Julian and Mal{\´y}, Pavel and Ji, Lei and Han, Jie and Moos, Michael and Marder, Todd B. and Bunz, Uwe H. F. and Dreuw, Andreas and Lambert, Christoph and Brixner, Tobias}, title = {Rapid multiple-quantum three-dimensional fluorescence spectroscopy disentangles quantum pathways}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, doi = {10.1038/s41467-019-12602-x}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-202529}, pages = {4735}, year = {2019}, abstract = {Coherent two-dimensional spectroscopy is a powerful tool for probing ultrafast quantum dynamics in complex systems. Several variants offer different types of information but typically require distinct beam geometries. Here we introduce population-based three-dimensional (3D) electronic spectroscopy and demonstrate the extraction of all fourth- and multiple sixth-order nonlinear signal contributions by employing 125-fold (1⨯5⨯5⨯5) phase cycling of a four-pulse sequence. Utilizing fluorescence detection and shot-to-shot pulse shaping in single-beam geometry, we obtain various 3D spectra of the dianion of TIPS-tetraazapentacene, a fluorophore with limited stability at ambient conditions. From this, we recover previously unknown characteristics of its electronic two-photon state. Rephasing and nonrephasing sixth-order contributions are measured without additional phasing that hampered previous attempts using noncollinear geometries. We systematically resolve all nonlinear signals from the same dataset that can be acquired in 8 min. The approach is generalizable to other incoherent observables such as external photoelectrons, photocurrents, or photoions.}, language = {en} } @phdthesis{Stahlhut2023, author = {Stahlhut, Philipp}, title = {Konzeption und Aufbau einer Nanofokus Labor CT Anlage in Reflexionsgeometrie auf Basis eines Rasterelektronenmikroskops}, doi = {10.25972/OPUS-30264}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-302648}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {In der vorliegenden Arbeit werden die Konzeption und Realisierung eines Computertomographen zur Materialanalyse auf Basis eines Rasterelektronenmikroskops mit einem r{\"a}umlichen Aufl{\"o}sungsverm{\"o}gen im Nanometerbereich diskutiert. Durch einen fokussierten Elektronenstrahl, der mit einer Beschleunigungsspannung von 30 kV auf eine mikrostrukturierte Wolframnadel mit einem Spitzenradius von bis zu 50 nm gezielt wird, entsteht ein kleiner R{\"o}ntgenbrennfleck {\"u}ber den mit geometrischer Vergr{\"o}ßerung hochaufl{\"o}sende Projektionen eines zu untersuchenden Objekts erzeugt werden. Durch Rotation des Testobjekts werden Projektionen aus verschiedenen Blickwinkeln aufgenommen und {\"u}ber einen speziellen Rekonstruktionsalgorithmus zu einem 3-dimensionalen Bild zusammengef{\"u}gt. Bei der Beurteilung der Einzelkomponenten des Ger{\"a}ts wird insbesondere auf Struktur, Form und den elektrochemischen Herstellungsprozess der R{\"o}ntgenquelle eingegangen. Eine ausreichend genaue Positionierung von Messobjekt und R{\"o}ntgenbrennfleck wird {\"u}ber Piezoachsen realisiert, w{\"a}hrend die Stabilit{\"a}t des R{\"o}ntgenbrennflecks {\"u}ber die Elektronenoptik des Rasterelektronenmikroskops und die Form der Quellnadel optimiert wird. Das r{\"a}umliche Aufl{\"o}sungsverm{\"o}gen wird {\"u}ber die Linienspreizfunktion an Materialkanten abgesch{\"a}tzt. F{\"u}r eine Wolfram-Block-Quelle ergibt sich dabei ein Aufl{\"o}sungsverm{\"o}gen von 325 nm - 400 nm in 3D, w{\"a}hrend der Quellfleck einer Wolframnadel das Aufl{\"o}sungsverm{\"o}gen der Anlage auf 65 nm - 90 nm in 2D und 170 nm - 300 nm in 3D bei Messungen an einem AlCu29-Testobjekt anhebt. Außerdem werden die Auswirkungen der Phasenkontrastcharakteristik der R{\"o}ntgenquelle auf die rekonstruierten Bilder nach Anwendung eines Paganin-Filters diskutiert. Dabei zeigt sich, dass durch Anwendung des Filters ein verbessertes Signal-zu-Rausch-Verh{\"a}ltnis auf Kosten der r{\"a}umlichen Bildaufl{\"o}sung erzielt werden kann. Eine Vergleichsmessung mit einem kommerziell verf{\"u}gbaren R{\"o}ntgenmikroskop zeigt die St{\"a}rken des vorgestellten Systems bei Untersuchung von stark absorbierenden Messobjekten. Das kompakte Design erlaubt eine Weiterentwicklung in Richtung eines nanoCT-Moduls als Upgrade Option f{\"u}r Rasterelektronenmikroskope im Gegensatz zu den weitaus teureren bisher verbreiteten nanoCT-Ger{\"a}ten.}, subject = {Computertomographie}, language = {de} } @article{ChristHaertlKlosteretal.2022, author = {Christ, Andreas and H{\"a}rtl, Patrick and Kloster, Patrick and Bode, Matthias and Leisegang, Markus}, title = {Influence of band structure on ballistic transport revealed by molecular nanoprobe}, series = {Physical Review Research}, volume = {4}, journal = {Physical Review Research}, number = {4}, doi = {10.1103/PhysRevResearch.4.043016}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-300855}, year = {2022}, abstract = {In this study we characterize the tautomerization of HPc on Cu(111) as a charge-carrier-induced reversible one-electron process. An analysis of the bias-dependent tautomerization rate finds an energy threshold that corresponds to the energy of the N-H stretching mode. By using the tautomerization of the molecule as a detector for charge carrier transport in the so-called molecular nanoprobe (MONA) technique, we provide evidence for an inhomogeneous coupling between the fourfold-symmetric molecule and sixfold-symmetric surface. We conclude the study by comparing the energy dependence of charge carrier transport on the Cu(111) to the Ag(111) surface. While the MONA technique is limited to the detection of hot-electron transport for Ag(111), our data reveal that the lower onset energy of the Cu surface state also allows for the detection of hot-hole transport. The influence of surface and bulk transport on the MONA technique is discussed.}, language = {en} } @article{DyksikMotykaKurkaetal.2016, author = {Dyksik, M. and Motyka, M. and Kurka, M. and Ryczo, K. and Dallner, M. and H{\"o}fling, S. and Kamp, M. and Sęk, G. and Misiwicz, J.}, title = {Photoluminescence quenching mechanisms in type IIInAs/GaInSb QWs on InAs substrates}, series = {Optical and Quantum Electronics}, volume = {48}, journal = {Optical and Quantum Electronics}, number = {401}, doi = {10.1007/s11082-016-0667-y}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-204672}, year = {2016}, abstract = {Optical properties of AlSb/InAs/GaInSb/InAs/AlSb quantum wells (QWs) grown on an InAs substrate were investigated from the point of view of room temperature emission in the mid- and long-wavelength infrared ranges. By means of two independent techniques of optical spectroscopy, photoreflectance and temperature-dependent photoluminescence, it was proven that the main process limiting the performance of such InAs substrate-based type II structures is related to the escape of carriers from the hole ground state of the QW. Two nonradiative recombination channels were identified. The main process was attributed to holes tunneling to the valence band of the GaAsSb spacing layer and the second one with trapping of holes by native defects located in the same layer.}, language = {en} } @phdthesis{Hausoel2022, author = {Hausoel, Andreas}, title = {Electronic magnetism in correlated systems: from quantum materials down to Earth's core}, doi = {10.25972/OPUS-25444}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-254444}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In the last decade continuous-time quantum Monte Carlo in the hybridization expansion (CTHYB) was one of the most successful Monte Carlo techniques to describe correlated quantum phenomena in conjunction with dynamical mean field theory (DMFT). The first part of the thesis consists of algorithmical developments regarding CTHYB and DMFT. I provide a complete derivation and an extensive discussion of the expansion formula. We generalized it to treat spin-orbit coupling, and invented the superstate sampling algorithm to make it efficient enough for describing systems with general interactions, crystal fields and spin-orbit coupling at low temperatures. But CTHYB is known to fail in the standard implementation for equal-time correlators, certain higher-order Green's functions and the atomic limit; we discovered that its estimator for the Greens function is also inconsistent for Anderson impurities with finite, discrete baths. I focus then on further improvements of CTHYB that we have conceived and worked on, in particular for f-orbitals and for taking physical symmetries into account in the calculation of the Monte Carlo observables. The second part of the thesis presents selected physical applications of these methods. I show DMFT calculations of highest accuracy for elemental iron and nickel and discover a new mechanism of magnetic ordering in nickel: the ordering of band structure-induced local moments. Then we analyze the stability of this phenomenon under pressure and temperatures, that characterize in the Earth's core. We find, that the mechanism survives these conditions and may give a significant contribution to the generation of the Earth's magnetic field. The next topic is the stability of double Dirac fermions against electronic correlations. We find, that the Coulomb interaction in the corresponding material Bi2 CuO4 are strong enough to destroy the double Dirac cone, and substantial uniform pressure is necessary to restore them. In the last chapter I derive the properties of Higgs and Goldstone bosons from Ginzburg-Landau theory, and identify these excitations in a model of an excitonic magnet.}, subject = {Monte-Carlo-Simulation}, language = {en} } @phdthesis{Hajer2022, author = {Hajer, Jan}, title = {Mercury Telluride Nanowires for Topological Quantum Transport}, doi = {10.25972/OPUS-29322}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-293222}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Novel appraches to the molecular beam epitaxy of core-shell nanowires in the group II telluride material system were explored in this work. Significant advances in growth spurred the development of a flexible and reliable platform for a charge transport characterization of the topological insulator HgTe in a tubular nanowire geometry. The transport results presented provide an important basis for the design of future studies that strive for the experimental realization of topological charge transport in the quantum wire limit.}, subject = {Quecksilbertellurid}, language = {en} } @article{FranichMederBehr2020, author = {Franich, Robert A. and Meder, Roger and Behr, Volker C.}, title = {Dewatering Green Sapwood Using Carbon Dioxide Undergoing Cyclical Phase Change between Supercritical Fluid and Gas}, series = {Molecules}, volume = {25}, journal = {Molecules}, number = {22}, issn = {1420-3049}, doi = {10.3390/molecules25225367}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-219327}, year = {2020}, abstract = {Conventional kiln drying of wood operates by the evaporation of water at elevated temperature. In the initial stage of drying, mobile water in the wood cell lumen evaporates. More slowly, water bound in the wood cell walls evaporates, requiring the breaking of hydrogen bonds between water molecules and cellulose and hemicellulose polymers in the cell wall. An alternative for wood kiln drying is a patented process for green wood dewatering through the molecular interaction of supercritical carbon dioxide with water of wood cell sap. When the system pressure is reduced to below the critical point, phase change from supercritical fluid to gas occurs with a consequent large change in CO2 volume. This results in the efficient, rapid, mechanical expulsion of liquid sap from wood. The end-point of this cyclical phase-change process is wood dewatered to the cell wall fibre saturation point. This paper describes dewatering over a range of green wood specimen sizes, from laboratory physical chemistry studies to pilot-plant trials. Magnetic resonance imaging and nuclear magnetic resonance spectroscopy were applied to study the fundamental mechanisms of the process, which were contrasted with similar studies of conventional thermal wood drying. In conclusion, opportunities and impediments towards the commercialisation of the green wood dewatering process are discussed.}, language = {en} } @phdthesis{Saxena2020, author = {Saxena, Sheetal}, title = {Multiwavelength Studies Of Gamma-Ray Emitting Radio Galaxies}, doi = {10.25972/OPUS-21538}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-215386}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2020}, abstract = {Although the contribution to the Isotropic Gamma-Ray Background (IGRB) from unresolved extragalactic objects has been studied for many years, its exact composition and origin are as of yet unknown. It is suspected that diffuse processes such as dark matter annihilation contribute to the total IGRB, as well as unresolved gamma-ray emission from Active Galactic Nuclei (AGN), including radio galaxies. Radio galaxies are a source class that emit strongly at radio wavelengths, some of which have also been detected at gamma-ray wavelengths by the Fermi Large Area Telescope (Fermi-LAT), and by very high energy gamma-ray Cherenkov telescopes. It is thought that due to the orientation of their jets, radio galaxies are detected less numerously at gamma-ray energies than blazars. Furthermore, only a small number of radio galaxies have been detected at gamma-ray energies though it is considered that others do as well. It is for these reasons that gamma-ray emitting radio galaxies, an interesting and elusive class of objects, are selected for investigation in this work. In order to reach the goal of better understanding diffuse processes, it is necessary to model the radio galaxy spectral energy distributions (SEDs). As AGN emission is variable with respect to time, it is critical to use simultaneously collected observations. Calculation of the SED based on simultaneous, multiwavelength data across the electromagnetic spectrum produces a reasonably accurate representation of the state of an object in a given time range. The gamma-ray emitting radio galaxies M 87, NGC 1275, Pictor A, and Centaurus A are selected here based on having been detected in very high energy gamma-rays by Cherenkov telescopes, as well as in other wavelengths. A uniquely consistent analysis approach is applied, in which each radio galaxy is analyzed the same way using simultaneously collected data. This approach sets it apart from other studies. Fermi-LAT raw data for each source in the sample is analyzed in time ranges which directly overlap the very high energy gamma-ray Cherenkov observations, as well as several other wavelength ranges. A synchrotron self-Compton (SSC) model is applied, which provides accurate treatment of synchrotron and inverse-Compton processes occurring in the jets of AGN, while estimating physical characteristics of the source. It is found that the spectra of M 87, NGC 1275, Pictor A, and Centaurus A can be well described by the same SSC model, producing values for the physical characteristics such as the doppler factor and magnetic field, which are relatively consistent with each other. In order to characterize the diffuse emission from dark matter self-annihilation, the radio galaxy SEDs are also fit with a dark matter model, resulting in an estimated dark matter particle mass of around 4.7 TeV which lies within predicted ranges. The highly dense regions near the black holes of AGN provide the optimal conditions for detecting these signatures. It is also found here that discrepancies between the expected emission and the observed emission in the spectra of some radio galaxies can be explained using the combined SSC and dark matter model. As emission from dark matter annihilation is expected to remain steady with respect to time, a key feature of this work is the novelty of the combined SSC and dark matter model, and the finding that dark matter characteristics may be revealed through similar multiwavelength analyses during future low emission states of the AGN. The radio galaxy sample is then extended to include all gamma-ray emitting radio galaxies detected by the Fermi-LAT, and a calculation of the core radio, total radio, and gamma-ray luminosities is followed through. A future step in extending this work would be to estimate the gamma-ray luminosity function of radio galaxies and their percent contribution to the total IGRB, based on the widely agreed upon assumption that a reasonable estimate of the gamma-ray luminosity function of a population can be attained by appropriately scaling its radio luminosity function, as gamma-ray luminosities and radio luminosities are strongly linearly correlated. This work has also provided the basis for such a calculation by outlining the theory and initial steps. It is the hope that the vast scope of the gathered data, its simultaneity, and the use of consistent analysis methods across the sample, will provide an improved foundation for a future calculation of the contribution of this population to the IGRB, as well as encourage stricter requirements for multiwavelength studies.}, subject = {Active Galactic Nuclei}, language = {en} } @article{BunzmannKrugmannWeissenseeletal.2021, author = {Bunzmann, Nikolai and Krugmann, Benjamin and Weissenseel, Sebastian and Kudriashova, Liudmila and Ivaniuk, Khrystyna and Stakhira, Pavlo and Cherpak, Vladyslav and Chapran, Marian and Grybauskaite-Kaminskiene, Gintare and Grazulevicius, Juozas Vidas and Dyakonov, Vladimir and Sperlich, Andreas}, title = {Spin- and Voltage-Dependent Emission from Intra- and Intermolecular TADF OLEDs}, series = {Advanced Electronic Materials}, volume = {7}, journal = {Advanced Electronic Materials}, number = {3}, doi = {10.1002/aelm.202000702}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-224434}, year = {2021}, abstract = {Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near-orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, 4,4′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)bis(3-(trifluoromethyl) benzonitrile) (pCNBCzoCF\(_{3}\)) is investigated, which shows intramolecular TADF but can also form exciplex states in combination with 4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting exciplex-based OLEDs additionally generate a sky-blue emission from the intramolecular emitter with an intensity that can be voltage-controlled. Electroluminescence detected magnetic resonance (ELDMR) is applied to study the thermally activated spin-dependent triplet to singlet up-conversion in operating devices. Thereby, intermediate excited states involved in OLED operation can be investigated and the corresponding activation energy for both, intra- and intermolecular based TADF can be derived. Furthermore, a lower estimate is given for the extent of the triplet wavefunction to be ≥ 1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, the findings allow to draw a comprehensive picture of the spin-dependent emission from intra- and intermolecular TADF OLEDs.}, language = {en} } @phdthesis{Balles2021, author = {Balles, Andreas}, title = {In-line phase contrast and grating interferometry at a liquid-metal-jet source with micrometer resolution}, doi = {10.25972/OPUS-23591}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-235917}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {As a non-destructive testing method, X-ray imaging has proved to be suitable for the examination of a variety of objects. The measurement principle is based on the attenuation of X-rays caused by these objects. This attenuation can be recorded as shades of intensity using X-ray detectors and thus contains information about the inner structure of the investigated object. Since X-rays are electromagnetic waves, they also experience a change of phase in addition to their attenuation while penetrating an object. In general, imaging methods based on this effect are referred to as phase contrast imaging techniques. In the laboratory, the two mainly used methods are the propagation based phase contrast or in-line phase contrast and the grating interferometry. While in-line phase contrast - under certain conditions - shows edge enhancement at interfaces due to interference, phase contrast in the grating interferometry is only indirectly measurable by the use of several gratings. In addition to phase contrast, grating interferometry provides access to the so-called dark-field imaging contrast, which measures the scattering of X-rays caused by an object. These two imaging techniques, together with a novel concept of laboratory X-ray sources, the liquid-metal-jet, form the main part of this work. Compared to conventional X-ray sources, the liquid-metal-jet source offers higher brightness. The term brightness is defined by the number of X-ray photons per second, emitting area (area of the X-ray spot) and solid angle at which they are emitted. On the basis of this source, a high resolution in-line phase contrast setup was partially developed in the scope of this work. Several computed tomographies show the feasibility of in-line phase contrast and the improvement of image quality by applying phase retrieval algorithms. Moreover, the determination of optimized sample positions for in-line phase contrast imaging is treated at which the edge enhancement is maximized. Based on primitive fiber objects, this optimization has proven to be a good approximation. With its high brightness in combination with a high spatial coherence, the liquid-metal-jet source is also interesting for grating interferometry. The development of such a setup is also part of this work. The overall concept and the characterization of the setup is presented as well as the applicability and its limits for the investigation of various objects. Due to the very unique concept of this grating interferometer it was possible to realize a modified interferometer system by using a single grating only. Its concept and results are also presented in this work. Furthermore, a grating interferometer based on a microfocus X-ray tube was tested regarding its performance. Thereby, parameters like the anode material, acquisition geometry and gratings were altered in order to find the advantages and disadvantages of each configuration.}, subject = {Phasenkontrastverfahren}, language = {en} } @article{RoedingBrixner2018, author = {Roeding, Sebastian and Brixner, Tobias}, title = {Coherent two-dimensional electronic mass spectrometry}, series = {Nature Communications}, volume = {9}, journal = {Nature Communications}, number = {2519}, doi = {10.1038/s41467-018-04927-w}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-226458}, pages = {1-9}, year = {2018}, abstract = {Coherent two-dimensional (2D) optical spectroscopy has revolutionized our ability to probe many types of couplings and ultrafast dynamics in complex quantum systems. The dynamics and function of any quantum system strongly depend on couplings to the environment. Thus, studying coherent interactions for different environments remains a topic of tremendous interest. Here we introduce coherent 2D electronic mass spectrometry that allows 2D measurements on effusive molecular beams and thus on quantum systems with minimum system-bath interaction and employ this to identify the major ionization pathway of 3d Rydberg states in NO2. Furthermore, we present 2D spectra of multiphoton ionization, disclosing distinct differences in the nonlinear response functions leading to the ionization products. We also realize the equivalent of spectrally resolved transient-absorption measurements without the necessity for acquiring weak absorption changes. Using time-of-flight detection introduces cations as an observable, enabling the 2D spectroscopic study on isolated systems of photophysical and photochemical reactions.}, language = {en} } @article{FritscheHoeckendorfAlvermannetal.2020, author = {Fritsche, Alexander and H{\"o}ckendorf, Bastian and Alvermann, Andreas and Fehske, Holger}, title = {Real and imaginary edge states in stacked Floquet honeycomb lattices}, series = {The European Physical Journal B}, volume = {93}, journal = {The European Physical Journal B}, number = {151}, issn = {1434-6028}, doi = {10.1140/epjb/e2020-10233-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-232347}, year = {2020}, abstract = {We present a non-Hermitian Floquet model with topological edge states in real and imaginaryband gaps. The model utilizes two stacked honeycomb lattices which can be related via four different typesof non-Hermitian time-reversal symmetry. Implementing the correct time-reversal symmetry provides uswith either two counterpropagating edge states in a real gap, or a single edge state in an imaginary gap.The counterpropagating edge states allow for either helical or chiral transport along the lattice perimeter.In stark contrast, we find that the edge state in the imaginary gap does not propagate. Instead, it remainsspatially localized while its amplitude continuously increases. Our model is well-suited for realizing theseedge states in photonic waveguide lattices}, language = {en} } @phdthesis{Kagerer2024, author = {Kagerer, Philipp Thomas}, title = {Two-Dimensional Ferromagnetism and Topology at the Surface of MnBi\(_2\)Te\(_4\) - Bi\(_2\)Te\(_3\) Heterostructures - MBE Growth, Magnetism and Electronic Properties}, doi = {10.25972/OPUS-36012}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-360121}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {In this thesis, a model system of a magnetic topological heterostructure is studied, namely a heterosystem consisting of a single ferromagnetic septuple-layer (SL) of \(MnBi_2Te_4\) on the surface of the three-dimensional topological insulator \(Bi_2Te_3\). Using MBE and developing a specialized experimental setup, the first part of this thesis deals with the growth of \(Bi_2Te_3\) and thin films of \(MnBi_2Te_4\) on \(BaF_2\)-substrates by the co-evaporation of its binary constituents. The structural analysis is conducted along several suitable probes such as X-ray diffraction (XRD, XRR), AFM and scanning tunnelling electron microscopy (STEM). It is furthermore found that the growth of a single septuple-layer of \(MnBi_2Te_4\) on the surface of \(Bi_2Te_3\) can be facilitated. By using X-ray absorption and circular magnetic dichroism (XAS, XMCD), the magnetic properties of \(MnBi_2Te_4\) are explored down to the monolayer limit. The layered nature of the vdW crystal and a strong uniaxial magnetocrystalline anisotropy establish stable out-of plane magnetic order at the surface of \(MnBi_2Te_4\), which is stable even down to the 2D limit. Pushing the material system to there, i.e. a single SL \(MnBi_2Te_4\) further allows to study the phase transition of this 2D ferromagnet and extract its critical behaviour with \(T_c \, = \, 14.89~k\) and \(\beta \, = \, 0.484\). Utilizing bulk crystals of the ferromagnetic \(Fe_3GeTe_2\) as substrate allows to influence, enhance and bias the magnetism in the single SL of \(MnBi_2Te_4\). By growing heterostructures of the type \(MnBi_2Te_4\) -- n layer \(Bi_2Te_3\) -- \(Fe_3GeTe_2\)for n between 0 and 2, it is shown, that a considerable magnetic coupling can be introduced between the \(MnBi_2Te_4\) top-layer and the substrate. Finally the interplay between topology and magnetism in the ferromagnetic extension is studied directly by angle-resolved photoemission spectroscopy. The heterostructure is found to host a linearly dispersing TSS at the centre of the Brillouin zone. Using low temperature and high-resolution ARPES a large magnetic gap opening of \(\sim\) 35 meV is found at the Dirac point of the TSS. By following its temperature evolution, it is apparent that the scaling behaviour coincides with the magnetic order parameter of the modified surface.}, subject = {Molekularstrahlepitaxie}, language = {en} } @article{IzquierdoKarolakPrabhakaranetal.2019, author = {Izquierdo, Manuel and Karolak, Michael and Prabhakaran, Dharmalingam and Boothroyd, Andrew T. and Scherz, Andreas O. and Lichtenstein, Alexander and Molodtsov, Serguei L.}, title = {Monitoring ultrafast metallization in LaCoO3 with femtosecond soft x-ray spectroscopy}, series = {Communications Physics}, volume = {2}, journal = {Communications Physics}, doi = {10.1038/s42005-019-0109-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-323265}, year = {2019}, abstract = {The study of ultrafast dynamics is a new tool to understand and control the properties of correlated oxides. By enhancing some properties and realizing new dynamically excited phrases, this tool has opened new routes for technological applications. LaCoO3 is one paradigmatic example where the strong electron, spin, and lattice coupling induced by electronic correlations results in a low-temperature spin transition and a high-temperature semiconductor-to-metal transition that is still not completely understood. Here, we monitor ultrafast metallization in LaCoO3 using time-resolved soft x-ray reflectivity experiments. While the process is entangled at the Co L3 edge, the time information of the different channels is decrypted at different resonant energies of the O K edge. Metallization is shown to occur via transient electronic, spin, and lattice separation. Our results agree with the thermodynamical model and demonstrate the potential of femtosecond soft x-ray experiments at the O K edge to understand correlated oxides.}, language = {en} } @article{HolzingerSchneiderHoeflingetal.2019, author = {Holzinger, Steffen and Schneider, Christian and H{\"o}fling, Sven and Porte, Xavier and Reitzenstein, Stephan}, title = {Quantum-dot micropillar lasers subject to coherent time-delayed optical feedback from a short external cavity}, series = {Scientific Reports}, volume = {9}, journal = {Scientific Reports}, doi = {10.1038/s41598-018-36599-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322485}, year = {2019}, abstract = {We investigate the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser when subject to delayed coherent optical feedback from a short external cavity. We experimentally characterize how the external cavity length, being on the same order than the microlaser's coherence length, influences the spectral and dynamical properties of the micropillar laser. Moreover, we determine the relaxation oscillation frequency of the micropillar by superimposing optical pulse injection to a dc current. It is found that the optical pulse can be used to disturb the feedback-coupled laser within one roundtrip time in such a way that it reaches the same output power as if no feedback was present. Our results do not only expand the understanding of microlasers when subject to optical feedback from short external cavities, but pave the way towards tailoring the properties of this key nanophotonic system for studies in the quantum regime of self-feedback and its implementation to integrated photonic circuits.}, language = {en} }